Nintendo has said that it won't support multiple gamepads at launch, but probably will in the future. As for performance with two, it was said that the frame rate on the gamepads will drop from 60 to 30. My belief is it will do a method of interleaving, in that for the odd frames, it will render from player 1's perspective and send to player 1. For the even frames, it will render from player 2's perspective and send to player 2. This way, there is no change to the amount of data that gets sent out wirelessly every frame.
So, if 3 were supported, it would be 20 fps per gamepad, and for 4 gamepads, 15fps.