If you see how much they've cut down the stuff the client needs to send to the server, you see how feasible it is to have that many players on their high end servers. The only thing that makes this different is you have more online enemies to shoot and more online allies to watch get shot at, so I reckon it should be good, despite all the client cutbacks.
Also, if you followed this game at any game event you would know that it is 2 teams, no restrictions or limitations. You can go anywhere you like, kill any enemy on the other team or heal any ally on yours. The maps are sort of divided into 4 at the start to stop spawn chaos but it just makes the fight in the middle so much cooler.
