In general, lots of people apply to ACGME programs but it's mostly comprised of FM, IM, peds, and EM. I really don't know exact numbers, I am making the assumption that GS is applied to similar to other more competitive specialties. Even though GS isn't that competitive for MDs or DOs going AOA, it is decently competitive for DOs going ACGME so less apply. It's an instant filter for less to apply. There are thousands and thousands more IMGS than DOs that's why I'm assuming there is a small percentage of DOs.
I agree it's scary business if you don't have a strong app - another reason not many apply, in my opinion. AOA is a strong and viable option, even for competitive students.
Good question and you're correct. The AOA match is before the ACGME match and if you match AOA you are bound to that spot and removed from ACGME match.