It all varies with the number of players that a school has try out for the teams. The school my son is at only has a rule not allowing seniors to be on the JV roster, but they're also no-cut for soccer. Teams can have as many players as they'd like on a roster, just as long as they don't have more than 18 active players for a given game. A couple years ago, our varsity team had a couple seniors who really just weren't very good - they practiced with the team every day, but were usually inactive for games. Occasionally, they were on the active roster, when we had injuries and such. And Senior night, they started all 11 Seniors, but subbed out a couple of the boys after a couple minutes.
Some schools around us have a large number of boys come out for soccer, so there's always cuts - and that's with 3 teams (Varsity, JV-A and JV-B). Other sports, needless to say, are worse, just due to the nature of the sports. For example, our freshman volleyball team apparently cut 14 girls - they took 10 for the roster out of 24 girls.