When the medical school expansion started in 2002, the aim was for a 30% increase in medical school spots (LCME). At that time, there were roughly 16000 positions, so the increase would lead to ~21000 positions in LCME schools. This was supposed to happen for the graduating class of 2015 (entering this year), but apparently they are only going to have accomplished a 21% increase by then (~19500), according to an article I read (don't ask where, I do remember that number though).
I'm not sure if you meant just LCME grads, but I'll add DO grads as well, who will number ~5000 in 2015. Together, thats ~24500 US graduates. If you count ACGME prelim (surgery + internal medicine) and ACGME transitional/prelim years, there are ~26000 positions in the match each year. Considering about 22,100 are categorical or advanced PGY2, that means ~4000 are transitional years. However, most who match into a PGY2 also need a PGY1 period, unless they are a previous graduate and are already in a PGY1 position. I don't know where to find that data...suppose all the people matching PGY2 also matched into PGY1, that's ~2700 spots less. So totally, there are ~23500 positions (categorical + preliminary).
So, you have 24500 US MD+DO for 23500 positions in ACGME match, but of course there are ~2500 AOA spots as well. So, you have 24500 US MD+DO students with ~26000 spots. (Again, because of the issue with the preliminary, categorical, PGY2 advanced, etc. these aren't exact numbers). However, this is assuming every US student will match and IMG/FMG don't match until after all of this, which we know isn't true. But unless DO schools stop expanding, there may very well be a time when US students outnumber the US residency positions. For now, though, there are "technically" enough spots for US students. We know no group matches at 100% though, so it will still get much tougher for all groups to match positions, with DO/FMG/IMG feeling more of a strain than US LCME students.
If I somehow completely messed these numbers up, let me know. They are not absolute numbers, and I honestly somewhat confused myself typing this up...so yeah.......