Actually, most schools will accept more people outright than spots they have available for their class because they know that some will turn them down. For example: if school X is a private school that enrolls 150 students, it will probably accept outright after interviews at least 250-300 people. Then the school will put a number of people on their alternate (or wait) list. The number of people on the list can never really be known. But the school will only take people off the alternate list when spots become open in the class. That is, in this example when at least 150 accepted people turn them down.
Now there are some schools that would accept a lot less for a class of 150 for various reasons, like good tuition, state school, or it is very "highly ranked" or is prestigious. These schools tend to get less turn downs.
So this means that there is no real way to tell the size of alternate lists in general and how many people are pulled off each year becuase it varries so widely by each school.