There have been years when no one comes off the WL. It's all a question of yield. Each year, USNA (as with other competitive colleges) puts out X offers of appointment, expecting that Y% of those will accept the offer. It's unknown if USNA creates some "padding."
So (FOR EASY MATH PURPOSES ONLY), assume that USNA wants an entering class of 1200. They know that historically, the yield is 90% (meaning 90% of those offered an appointment actually accept). They make offers to 1330 candidates (which, if 90% accept yields a class size of ~1200). If fewer than 1200 accept, they start taking people off the WL until they reach 1200. If more than 1200 accept, they have a problem (as happened about a decade ago) and definitely don't take anyone from the WL. Unlike civilian colleges, the maximum number for an entering class can't be exceeded.
They can "pad" these numbers by (in my hypo) giving fewer than 1330 candidates offers -- going for less than the usual yield to hedge against too many acceptances. So they might only offer 1310 appointments. If they get the expected 90% yield, that allows for a decent number to come off the WL. If they get >90%, they are still ok.
Of course, it's more complicated than the above. There are LOA folks waiting on a medical waiver that factor into it. Some people who accepted and now decline b/c they got accepted to a preferable school. Moreover, we don't know how USNA treats the yield in terms of the number of offers they put out and this approach could vary from year to year.
Once they determine how many acceptances they have, they know whether they take anyone from the WL and, if so, how many. Medical or other last-minute issues can lead to offers of appointment very late in the cycle.