USNA Admissions has said that, some years no one is taken from the WL. In other years, there are anywhere from a handful to more than a dozen. As Hoops has repeatedly said, it depends on yield and USNA's yield has been very high in recent years. If the class size is already 100% of what they want, then USNA will send TWEs to almost all of the WL but may still hold onto a few WL candidates in the event someone has a late-breaking medical issue or other reason that prevents them from joining the plebe class. Or if someone is waiting for a medical issue to resolve (i.e., successful recovery from surgery) and it doesn't happen in time.
Bottom line: every year is different. The fact that USNA took 20 people from the WL in some prior year (if that has ever happened) has absolutely no bearing on whether even one person will be accepted from the WL this year. So, as Hoops has said, if you're on the WL, you should assume you will receive a TWE and proceed accordingly with your alternative plans. If you do receive an appointment, then you need to decide what to do -- but definitely don't count on it.