How many people would have to be in on The Conspiracy (see, I gave it caps)

if it were really true?
I chose the Limbaugh comparison merely to demonstrate the kind of puzzled bemusement with which I regard people who believe the Earth is flat, tis all...I'm not disagreeing with your right to believe whatever you want, I just find it endlessly, curiously fascinating...
How do people come to believe such a thing? It's obvious how people come to believe the Earth is round; it is, after all, the accepted "normal" belief...but such a huge huge deviation from the facts...how do you come to arrive at such a worldview?