There isn't an upper limit to how many people can be in a Roll20 campaign - there are some seriously large campaigns (100+ people) that work as communities instead of just games. Google Hangouts has a limit of 10 people. You can weed players out by either kicking them from the campaign - in the campaign's main page (before you launch the tabletop), hover over the person's icon with your mouse and then select "kick player" when it appears. Having run sessions with eight players, it's much harder and more chaotic to do online than in person. So, I'd stick with no more than five or six until you get the hang of that and then possibly add more down the road. Also you might want to consider using recruitment forums to pre-screen people and only let those in to the game who you want playing.
