Would 9/11 really need that many people to stage? The actual plot was executed by a fairly small number of people; surely faking it wouldn't take more than an order of magnitude more, especially when you control much of the process of investigation.

I don't believe it was a conspiracy, but I also don't think that there's a fundamental reason why it definitely couldn't have been.

I remember an estimate was that it would require 7,000 people.