Apple are really annoying for people that need up to 8 players
Generally you are going to want to use a server when you get to that number of players anyway. Gamecenter isn't designed to handle large scale multiplayer. It is designed for a few people to connect devices and then shovel the networking information to every client. This is all good for a small number of devices but as it grows the amount of data increases quickly to unmanageable without a server to distribute it in an intelligent way.