The number of people you'd need would depend on who joins. A bunch of unarmed and poorly motivated people with disparate agendas join, you're gonna need a lot of people. The IT dept of the Pentagon joins, you need a lot less people. People in mainstream media, banking, utilities, energy, state/local govts join, that can make a huge difference.
I'd like to think that our 3 million volunteer citizen soldiers would refuse to "go to war" against US civilians, even civilians behaving badly. But things like this have a way of starting from something isolated that didn't seem significant, and snowballing quickly before anyone realizes its gotten out of hand.
** Political sig removed **