In 97-98 (the latest figures I have), USC had about 6100 applicants.
They interviewed 700 that academic year. In different years the number of interviews may lie in the range 600-800.
After interview, candidates are placed into three categories: accept, wait-list, reject.
Class size: 150
After the first 150 acceptances are sent, the wait list is loosely prioritized into several categories. There will be a latent period because about 50 of of those initially accepted, who are waiting for acceptances from other medical schools will later accept those offers, leaving 50 seats open for wait-listed candidates. Some accepted elsewhere do not relinquish their places until they are given a deadline, commonly June 15, to say yes or no, even though some already know they are going elsewhere, or will sit on their hands until the very last minute to make up their minds. That is why the greatest movement from the wait list is after June 15, a common deadline date set by AMCAS rules.
Your likelihood of getting accepted from the wait-list depends how near the top of the list you are. Since USC does not encourage phone calls, it is likely they want to keep that information close to the vest, probably because candidates' positions on the wait list may be somewhat fluid. When an accepted candidate withdraws, the spot may be filled with someone similar, say a female for a female, to keep whatever kind of balance in the class they have in mind. I am guessing on that last point with respect to USC, but it wouldn't surprise me if many medical schools also do that. After all, it is impossible to absolutely clearly rank one candidate higher than another because of, for example a GPA difference of .10, or an MCAT score difference of 1, or even more subjective variables. How medical schools that do rigidly rank wait lists do it, I don't know and I can't even imagine.
On that last commment, I am reminded of a paper that once appeared in a journal. Yale medical school did a follow up study on what happened to applicants they interviewed, and might have accepted, but who did not make it into the class. The conclusion: had they filled the class with those on the wait list, eventually turned away, and who found places in other medical schools, the class would have done just as well. This underlines the vagaries of admission decisions from school to school and from year to year. However, the extreme conclusion is not warranted that if the names of all applicants, or even just those interviewed, were placed in a barrel and a blindfolded committee chair was to pick names at random, the otcome would be as good as a more selective method. In any case, absolute precision in making choices is an impossibilty. And adcoms know that. This is true of any competition, such as beauty contests and Olympic iceskating, or non- competitions such as trial by jury and the grading of essay exams, in which fallible human judgement determines the outcome.
Maybe you should propitiate Fate with a burnt offering, or kill off the competition, as Alec Guiness did in an old comedy film, Kind Hearts and Coronets!
Initially, 150 offers are made