Games : How do i testfor a certain amount of players in a certain radius

So i am making a spleef game on my server and i need to testfor 12 players within about 35 blocks from the command blocks.

I tried /testfor @a[r=35] leading into a tp command to tp the players into the spleef arena. what i dont have is to make it only tp 12 players and not any more or less. So if you can help me with this i'd appreciate it. thx