The AAMC publishes data on matriculants to US medical schools. Looks like there is some data on how many international students are accepted at US medical schools:
Looking at Table A-5:
Out of 1320 applicants whose state of legal residence was outside US, only 124 matriculated to a US medical school. Thats a matriculation rate of 9.3%. Certainly not great, but its actually higher than what I was expecting.