I have recently encountered teams sending multiple players onto the field with a captain’s armband on. It is my understanding that there should be one captain per team, per match with an armband.
USSF answer (March 15x, 2009):
While the traditional number of captains per team is one, there is no limit in the Laws of the Game on the number of captains a team may appoint for each game.
However, as a practical matter, when a team sends more than one person out for the coin toss, the intelligent referee will ask who is the MAIN captain — the one to speak with if there is a problem (and that is the only one who should be recorded in the referee’s notebook). This is not American football, and we must distinguish between coming out for the coin toss and being officially recorded as THE captain. There should be only one player per team who is THE captain.