Usually, many.
Remember the purpose of the WL is to ensure that the school will have a pool of applicants who can fill any of the holes that MAY remain after accepting applicants have made their decisions. Here are several reasons the list needs to be long:
From the school’s POV, they don’t know if someone they put on the WL will want to come to their school. Maybe that student already has offers at schools they love and maybe those schools have been very generous with $. Also, there are often students who start out on a WL, but after attending revisits, they realize they are really happy with an option they already have.
They need people in the pool to fit wherever they come up short, and it could be in one specific group. So they need lots of back-up in every category - day, boarding, girls, boys, athletes, musicians, math and robotics, international, geographic, various diversity, etc.
So if the shortfall is domestic, boarding girls and they need 5, they may need to have offered WL positions to 30 or more to be able to get there. And of course, they already have 30 or more day gorls, 30 or more domestic boarding boys, international girls and boys, etc…
Nobody knows at this point where the school will come up short. Sometimes, after revisit days start, they start getting a little more clarity, but it’s not until after A10 that they know for sure.
Sometimes too, schools will let some WL students know that they will not be going to their WL. If you get this news, it means they have filled the buckets you fit. But if you don’t while others do, it means that they still don’t have certainty. Overall, my advice is to focus on the options you DO have.