How many? Again, most the DOs that match MD either A. Have something that links them to that program like doing years of research there or B. Habe special in at that program. It's not worth the squeeze. What I mean is, i would not apply only MD because there is a 99% chance you will NOT match. Sure, apply to all the DO programs and then however many MDs you want, but your eye should be on the DO programs.
You would be surprised at how many auditioning students don't understand statistics and go unmatched because they rotate at 1-2 programs, rotate at 6 programs, but of those 6, 3 are MD, or just purely rotate at MD places. If you are serious about matching ortho, you need to use common sense stats and apply where you have the best shot (which is DO ortho at places you ROTATED AT).