You and one other person can play over LAN using the same copy. However, in a game of 10, for instance, 5 copies would be required. Also, to play on ICO(Ironclad Online) each person must have their own copy.
-Twilight Storm
-Edit: Also, note that once the game is installed you do not need to have the disc in the drive to play, it is completely installed on your computer, along with any expansions you have.