I guess it depends on your stats and experiences and stuff. I will say that I was very surprised how my cycle went. A few of the schools that I thought I had no shot at, showed me some love. The schools where I really thought I had a shot at because of location and my secondaries and stuff, gave me nothing. The whole thing is a crap shoot. Best to play the numbers game if you have the money to spare. After you have interviews and acceptances, then you can be picky.
I have a 3.6, balanced 28 MCAT, a crap ton of research experience, some clinical experience, good shadowing, a DO letter, etc. I applied in June to 10 schools ( a mix of top, mid and low tier DO). By september I had 5 interviews and by October I had 2 acceptances in hand - one to a very reputable old school, one to a new school. I took the acceptance to the good old school and withdrew my application and called my cycle done.
So take that for what its worth. If you have better stats and experiences then you can apply less. If your stats are worse then maybe apply to 10 or more. In hindsight I would have only applied to 8 schools - 2 of the schools on my list I would never have attended under pretty much any circumstance (except if they were my only acceptance). I consider myself a very average applicant (stat wise) so you can use my example to judge where you are at.