Let us define "in on it."
If "in on it" means knowing there's something stinky going on in Building B, then I suppose that several tons of people would have to be "in on it."
If by "in on it" you mean in possession of guilty knowledge and verifiable proof? Then, no. The scullery maid is not "in on it." She just knows something funny is going on, and she'll be better of minding her business.
Tons of people having to be "in on it" is not only a false objection, it's a ubiquitous objection - but let us look at a conspiracy where this objection is actually TRUE - where "a ton of people" actually DID have to be "in on it."
The D-Day invasion. Tens of thousands of people were "in on it." They fooled the nazi war machine and intelligence apparatus.
So - what were you saying about how many people have to be "in on it?"