This number varies from school to school and from year to year—sometimes hundreds will be admitted, but sometimes none will. Harvard's dean of admissions, William Fitzsimmons, told The Crimson earlier this year that in recent years, Harvard has accepted between zero and 228 waitlisted students.
The number of students taken off a waitlist is connected to a school's yield, which is the number of admitted students who decide to matriculate. Schools know that not 100 percent of students they admit will say yes, and they plan for this. Still, if fewer students matriculate than expected, they start taking students off the waitlist.