No, there is no limit specified anywhere in the Teamwork rules for how many people can work together. Yes, even mooks can gang together to Fight at +10. Ten mooks would in fact attack at +11: one mook's +2, and the other nine adding +1 each.
It is up to the players of a game to moderate this and artificially control the difficulty.
If you're concerned about a +10 attack, though, bear in mind: if you've put a single person up against that many mooks, that person is already in a very dangerous situation and the conflict might not go their way anyway.
The Fate rules wouldn't specify such a rule anyway.
It's not too surprising they don't do anything about this. There's two reasons why.
1. It would just get in the way of legitimate play.
The books offer you plenty of guidance on keeping things sane and fun, but if you want to run things at a higher power level, which you can, any attempt by the rules to limit things quantitatively would just be disruptive to peoples' attempts to do that.
If you wanted to run a large gang fight without making a hundred dice rolls, or have a dozen people gang up against someone who's singularly powerful (or singularly unfortunate), any arbitrary upper limit set by the Teamwork rules would only get in the way. And you can do this if you want to.
If you don't want to be in that circumstance, it's your responsibility as players to not create that circumstance in the first place.
2. The narrative, not the rules, should help you keep things sane.
Fate is not a rules-first game. The rules are pretty important, but they're not the entire system. If you're in a situation where you're wondering whether 50 people should be able to gang up to Overcome a steel door, you shouldn't be looking to the rules to help make sense of this.
Consider the narrative, and whether it makes narrative sense for those 50 people to gang together and help. Maybe they can — the door is big enough, they've got enough space, they're using a giant battering ram — or maybe they're in a tiny cramped passage and past a certain point, extra people isn't going to help.
Also, consider whether there's any fun value in stopping them or saying no.