I don't like how this post generalizes. 2531 regular americans is not the same as 2531 Nazi officers who will have their families turned into soap if they don't keep their mouth shut.
The whole "loose lips,sink ships" mentality of WW2 is a very interesting case study where entire towns were in on a secret. Lots of other shady things were conspired by governments and inteo agencies from the 50's through 80's and lots of people were in on it.
Merely establishing prolonged social contact with an ex-military person is all it takes in my experience for me to be told of some conspiracy they've seen that I wouldn't believe (I don't even care!!).
A lot of the famous conspiracy theories are false flags but they have some root in reality. If someone told you have the stuff snowden leaked without proof would you not shrug it off as baseless conspiracy theory? Exactly how many people were in on it? His leaks happened in 2013 but they were from 2008!
In short my opinin is that the number of people you can include in your conspiracy largely depends on their incentive and intent to keep their mouth shut as well as your ability to mount false-flag misinformation campaigns before and after the fact.