NHL roster size is 23 players, but only 20 players are allowed in any game. A normal game roster includes 12 forwards, 6 defensmen, and 2 goaltenders. The other 3 players are "scratches" and cannot participate in that game. Their purpose is to fill in if another player becomes ill or injured and is unable to play. These roster swaps are not allowed during the game though, only before the game starts. The only exception is if a teams starting and backup goaltender are both injured during a game and can't continue playing. In this case, the team is allowed to find any person they choose to serve as the emergency goaltender for the remainder of that game.