http://oilers.nhl.com/club/news.htm?id=430807
"Beyond those limitations, the league does not dictate the make-up of a team's roster, though the standard make-up used by most teams for the 20-player game roster is to dress 12 forwards and six defensemen along with the mandated two goaltenders. Rarely a team will use 13 forwards and five defensemen, but it's not overly uncommon for a team to dress 11 forwards and seven defensemen."
the link above has a great article that explains the 20-player "dressed list" for games, a 23-player active NHL roster, a 50-contract maximum, and a 90-player maximum reserve list.
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Each team can have 25 players on the roster at one time, but they often have 40 players on the team.
generally, in the NHL, 18 skaters and two goaltenders are dressed for a team for a game. usually 12 forwards and 6 defenseman make up the 18 skaters.
I would say 2 per team; This way if one can't play or is unable to, you have a backup; or simply if one gets tired, the other can take his or her place.