The roster page has always said it only shows the first 30 players, so if there is a limit (not sure there is) its ridiculously high.
2 or 3 players at each position is pretty standard, especially since you want guys who can play multiple positions. Most teams usually use 12-15 guys. The key is how many can you play and still maintain their game shape. Too few players and they play too many minutes and get tired, too many players and some rarely play and likewise suffer.
So for 3 games per week something around 13 players seems to be about the sweet spot for getting your key players the minutes they need to train without over-extending anyone. For each position I would recommend a starter and a back-up, and then round out the roster with 2-4 guys who you don't mind playing a few minutes here or there at whatever spot.