Depends on the school, but I ran a few calculations on most of the schools in Texas (UNTHSC UTSW, UTMB, TTUHSC, Baylor) based on interviews given in the previous year. The average was 33% chance for acceptance if given an interview.
Kind of a round about way of answering your question, but it seems like most schools receive ~1500 applications, some with 2000+, and then roughly 150-200 are invited from that pool, so roughly 10% chance of getting an interview. Of course, this statistic is greatly skewed due to proportions of students that submit to schools without meeting the minimum requirements and are automatic toss outs (but not all schools function that way.)
To sum this up: there isn't a safe way to calculate how many interviews a school will give, just an estimate based on the year before. But you can make safer predictions (sans the estimated number of interviews given) of the chance of getting an acceptance after interview because of set number of seats and all are qualified.
I'm not sure I answered your question, but hopefully something I blathered helped.
