Well if all the Quidditch teams have 7 players (3 Chasers, 2 Beaters, 1 Keeper and 1 Seeker) then there is about 28 but they also have subsitutes so add two for each then there is about 36 players all up in the Hogwarts teams. (7 on each team with 2 subs = 9 players each team)
Substitutes would be practical. But there is no mention of substitutes. That was a problem in the fifth year when Umbridge banned Harry, Fred and George from the team. Angelina had to replace all of them. She had no substitutes to draw from.