MLS trading cards challenge

I read this evening on the Sports Card Forum that Upper Deck is going to be producing a 100 card set representing the 2007 MLS season. Sounds cool but why only 100 cards in the set. Assuming each and every team had only 11 players that ever played, 13 teams in the leage, works out to be 143 starters when each team plays a game.

So how does the challenge work?