The amount of letters vaires as far as I know...but to cover your butt...have a few from science faculty (or pre-prof committee), one non-science faculty, and one D.O. (MD can be subbed @ some schools...but DO is highly reccomended...and a D.O. letter shouldn't be too hard to get with a day or two of shadowing)...
The AACOMAS PS is 3000 Characters, or about 500 words.
One thing though, I think that the first step would be to submit the primary and work on the PS...then work on the LORs...they will do you no good if you do not get secondaries.