That depends on the league settings, the site should tell you either during the registration process or after you have the team set up. (it may say # of roster spots or something along those lines.)
Once your setup, let us know if you have other questions, you've found a great place for information!
"We couldn’t do diddley poo offensively, we couldn’t make a first down, we couldn’t run the ball, we didn’t try to run the ball, we couldn’t complete a pass - we sucked." ~ James Ernest Mora