OK, short answer: Don't worry so much. That's a largely unnecessary concern because Journeys don't really need to be balanced by level in that way. They're not a CR style situation, due to the mix of positive and negative outcomes, most of which are temporary bonuses or setbacks.
Here's the REALLY LONG ANSWER:
Ok, there's a danger with any underlying assumption that Journey events are all negative, or equivalent to combat encounters. Bear with me as we embark on another epic reply.
When considering Journey Events I'd be wary of inadvertently applying the same logic on balance as one would to wandering monsters or combat encounters in a game. (Wherein too many, or too difficult an encounter = party death spiral and unfun times for all)
Combat encounters need firm regulation because they are highly complex sets of interactions, that can kill characters, and have wider reaching consequences. The positive outcome is largely null: player survival is the aim.
Ups and Downs, not just Downs
On a Journey, the company is as likely to encounter something that mechanically helps them, as they are a battle or something that penalises them. They might find a great campsite or simply encounter an interesting NPC. They might see a mechanically inspiring sunset.
Or they might run for their lives from an unbeatable foe and gain shadow. Or stumble into a place that makes them feel weird.
Most events offer a combination of possibilities, within each event itself, and most have an option for a positive outcome. That's highly significant when it comes to balance.
This is quite different to random monster encounters, which offer survival as the sole win state, mechanically speaking. (And the distant reward of XP, as do Journey Events) They might offer treasure, but in Middle-earth, that's a whole other kettle of fish to vanilla 5e games.
Anyway! Many Journey Events offer the potential for much more directly positive outcomes than survival as their win state.
This means that they need a different kind of treatment to random monster encounters when assessing how many to use. And when using a series of Journey events you're stacking up the opportunity to gain and to lose, not just the opportunity to lose.
This is much more directly observable and steerable than combat with all its mechanical abstractions, instantly-inflicted, serious problems, and knock-on effects over a much wider time scale. We need a system like CR (with all its foibles, and if we're being realistic, huge need for... wait for it...LM discretion*) to manage combat challenges.
By providing the possibility of mechanical "buffs", a great deal of the direct concern on balance is mitigated with Journey Events. (It's not *quite* that simple but hey close enough)
If you deploy "too many" (which is a tricky thing to define in itself, because it relies heavily on LM intention) randomly-generated events, there's a chance the built up swing towards positive or negative outcomes might be too great (see below). That's true. But it's not *quite* the thing that's being raised as a concern about balance.
The one part that isn't quite so happily loose is the lack of Long Rests. I hope that's really explicit in the rules as they play out at the table, and something that is very clear in play. The big challenge mechanically to Player-heroes is that lack of a Long Rest. That's the part that's grinding on them, and it should be obvious and ever-present in play, rather than a hidden ambiguity that would be easy to miss.
This alters some of the underlying calculations you may be used to, but in our experience was never forgotten by players, so isn't too hard to work with.
All that said, you are definitely going to have to play with the Journey rules a few times to really understand how best to deploy them. Guilty as charged. But I certainly believe the outcomes are worth that learning curve.
Numbers of Journey Events and what that means
I agree that there is a definitely an ambiguity around whether scripted events count towards the total number of things that happen on the road. That isn't categorically spelled out, and falls to the LM to decide in their own game. Because ultimately that's a net gain in play.
What that actually means mechanically is less obvious, however.
If the working assumption is that the Journey rules are designed to offer a finely balanced set of predetermined challenges, something like CR, and assuming that altering the number of those occurring breaks the system, sure, that would clearly be bad.
But that isn't quite how the Journey rules shake down. They're presenting a much looser framework of opportunities for potential outcomes, both positive and negative.
Given the way the tables and events themselves are balanced, the consequence of too many or two few Journey Events isn't about mechanical balance as we normally come across it in "encounters" meaning "combats".
Due to those positive as well as negative potential outcomes, one or two "too many" won't mechanically unbalance a game in any critical sense. You might give your players advantage AND inspiration AND a level of exhaustion. You might give them three levels of exhaustion. But that all happens over a far slower time scale than combat, both in and out of character.
There are so many footholds for the LM (and indeed players) to steer Journey events that it would be a curious kind of game where the journey got all that wildly out of control, mechanically speaking. That ambiguity that offers areas of Loremaster discretion is deliberately built into the framework as a feature. In part due to the really large slice of thematic concerns that Middle-earth invokes.
Control of precisely what occurs in the most potentially negative events (i.e. Combat) is left to Loremaster to decide. Which, one could argue, relies on LM ability rather than a strictly mechanical basis. Guilty as charged. Whether it's an encounter with two wounded Goblin scouts in broad daylight or 100 in a cave is up to the Loremaster. That, on balance, where we're trying to make a setting that pleases the broadest number of gamers, is a feature.
However, one could equally make the case that this loose approach and the distinct possibility of positive outcomes in events could result in Journeys actually buffing a company, especially with a combination of a fortuitous embarkation roll, some luck in the events rolled, and similar luck in the arrival roll. Non-coincidentally most of the benefits available are temporary.
Worst case scenario = wandering monsters
Looking at the other direction - Rolling really badly, in multiple events, could result in a *really* terrible time on the road. Quite deliberately. The upshot of this is potentially more serious and long lasting - death can certainly occur in a randomly generated combat.
This is certainly the place where a combination of "too many" Journey events, poorly matched to the company, with unlucky dice rolls, and insufficient LM discretion in how results are applied, could hurt a game. A safe bet would be to stick to the number of Journey Events in the rules. Don't add any. I'm happy that's implicit in the rules. If you find yourself wanting to add more mechanical events for a reason, then you can do so. That's explicit in the rules.
However, this potential fail state of the Journey rules, when it punishes the company the most, where all the rolls have been unlucky, and when only negative outcomes come up, especially combat encounters, places it on a more or less even footing with random monster encounters: an inexperienced LM might easily randomly generate too many foes, might make a poor assessment of what to throw at the company. Dice rolls can go badly, the LM might be unresponsive or make mistakes, and as a result, characters might die or players have a bad time.
That's the worst case scenario for Journeys - that at their absolute worst, they acquire the dangers of a wandering monster table run by an inexperienced LM. Which is (arguably and unkindly, but using the same logic) the default state for a vanilla 5e game.
What is a scripted event?
Adding in scripted events in addition to randomly generated ones, as we see in WA, has another feature worthy of brief examination - scripted events referred to in the LMG don't necessarily contain any element of mechanics, and so don't fall under a CR style situation. They can simply be plot events that advance the story, and don't contain challenges that hinder or help player-heroes. That's significant when it comes to talking about "how many" is too many, in any definitive sense.
Are the Journey rules a Swiss Army knife? Sure.
Do they require some experience to make them absolutely shine? Sure.
Are there some areas of ambiguity? Yes.
Do they need more regulation in case someone uses them wrongly? We don't believe so.
Are they too tricky for an inexperienced LM to run and have fun with? We've seen it done many times, so we don't believe so.
Can our scenario writers exploit/stretch bend the rules as written if they believe the challenge is about right for the scenario they're creating, and it works in context? Absolutely. It's not a strictly mechanical thing.
Can we put down a rule that gives you that level of knowledge and authority? Maybe. But we probably won't.
So on the flipside: The Swing
Here's where I think the Journey rules have an issue to watch out for, in relation to how many random events from the table you might choose to add.
Let's hugely simplify and say a Journey event offers this outcome:
+1, 0, or -1 (where a positive number refers to a positive outcome, and a negative a negative outcome)
So two events offer:
+2, +1, 0, -1, -2
Three events offer:
+3, +2, +1, 0, -1, -2, -3
And so on.
And those outliers get quite swingy. They aren't *likely* to occur. It's statistically unlikely that those higher outlying numbers will actually show up. Presenting the numbers this way makes it look far worse than it is.
But it is still possible.
Some of those bonuses or penalties are temporary, or noncumulative. So it's not quite this "bad". I'm using a very broad brush here.
The Embarkation Roll also moderates this a little too.
And ultimately, the big pluses are things like Inspiration, Advantage on a roll, stuff like that. Nothing that will spoil your game and the company will have earned those bonuses. The rules were very deliberately designed that way.
The negatives are potentially more harmful. Exhaustion is cumulative. And it has wide reaching effects. One type of negative outcome is battle, which can be lethal. Especially if you've had a very poor run of luck.
Equally, consistently offering a series of direct 0s is bad. (This is different to getting +1 and then -1. I refer here the very slim possibility of no outcome, in a series of encounters) Probably the worst thing that can happen, although the company might be relieved. I don't like a lot of mechanical effort to be put in for nothing to happen. Which is why the Journey rules aim squarely to avoid that. I pondered whether to include that 0 result here, because it's rare to generate a null state with the rules. But it can happen, so it should be included.
So the journey rules potentially do rely on the dice throwing up a nice spread of numbers over time. It's not a deal breaker if they don't. But it's better if they do. That's a potential weakness.
And if you add in a *lot* of randomly generated events, you're potentially opening up the possibility of a wider spread of randomness. I wouldn't personally recommend adding random journey events - scripted ones? Sure. Random ones, less so.
This effect is mitigated over several journeys. (One reason we mixed up how Journeys are implemented in WA is to keep it fresh, and show some different styles) It's not the end of the world by any means to have a strongly positive or negative outcome to a Journey. It can be fun, and a feature in either direction. And I don't think we should have built in any mechanical chokes on that. But it's there.
* I could talk all day about rules that pretend or appear to offer mechanical certainty on the page, but don't offer anything like that in play. And how happy I was to see how much of that 5e embraced. The difference between UI and user experience greatly interests me.