To play the Magicka campaign in co-op mode, do we need to buy a copy each, or is one enough?
You can play locally with one copy of the game, one person playing on keyboard and 3 on gamepads.
According to the Wiki:
Magicka is rather unique in its handling of multiple players. Up to 4 players can either connect via a network, or directly on one machine using a USB Gamepad.