Almost every sport has a limited roster. Only X number of players are on a basketball team, Y number on baseball, Z number on football, etc. Those numbers are usually set by the league in which the school participates, although it could vary by state (I'm not sure).
In our high school, only a few sports are unlimited - cross country in the fall, indoor track/field in the winter, and spring track/field (outdoor). Some coaches in some schools may have tryouts and limit the participants, but ours very deliberately does not do that. Many events just run extra heats if there are a ton of kids - the more "elite" runners will be in the first heat, but often subsequent heats have the next few tiers of kids. There may be some events that have a qualifying time/distance, but there's usually something else where everyone can have a chance. Cross country works the same way - the less experienced or accomplished runners will be at the back of the pack, but they still run the same course. In our school, everyone (from state championship winners) to the next to the last kid, wait until the last kid crosses the finish line, cheering them across. In track, kids who aren't in a few events work as timers or other assistants - vital team members.
Some schools, if large enough, may have some club sports - but that will vary based on the availability of fields and courts. Rec leagues vary by town, and there are private travel leagues that can be very competitive. So it varies.
Your best bet is to talk to the athletic director to find out if there are orientation programs for new students and parents, or the rec department director or other staff member who supervises those teams.
ETA: I just read Julie S's response and went to check your other posts. I have to agree with her in asking why you are pursuing this. Your children aren't sporty, you say. So why do you care about high school sports? If this is about scholarships, forget it. This is NOT how your child is going to go to college, no matter how little money you have. Do not, DO NOT push them. If this is about having your kids be physically active in some way, for general health and non-competition, fine - find something totally recreational, or consider XC/track - where they only try to improve on their prior time, not beat others.