I think I read this somewhere(probably The D.O.'s by Norman Gevitz) that by 2020 we may see roughly 20~25% of practicing physicians and surgeons hold D.O credentials. It's actually really fascinating how fast these numbers have increased within the last recent decade(or two).
If it ever does get to a point where there are too many schools, the LCME will most likely absorb all the osteopathic schools and phase out COCA. COCA as a accrediting agency is a joke in comparison to the LCME.