Who can afford two controllers to play a single game. Thats eight controllers for four player, insain. Though I have to say it does look like it can do everything the Wiimote with Motion Plus can do. But I'll stick with my Wiimote. Notice that at E3 2006 Nintendo showed tons of games where you would use two controllers, then they dropped it. Sony should have taken note!
actually, 4 is the most you'll ever need.
Only two players can play with 2 move controllers each, and up to 4 players with one controller each. So 4 is the max. I think the navigator or whatever it's called will support another 4 as a supplement.