It's not published. In general the ratio of applicants/seats is about half of that of the ratio of applicants/seats for MD programs. However, in general the stats of MD/PhD applicants are higher.
Don't get intimidated by low numbers of seats at programs. Sure a MD/PhD program with 10 seats may get 300 applicants. But then again, a MD program with 100 seats may get 3000 applicants. If you have good stats and research experience, go for it. I believe that MD/PhD admissions is much less of a crap shoot than is MD admissions.