In California, apply to all of the programs. Interview at your top 4-5. Rank them all. I know of a good, non-psycho candidate with extremely high scores who matched at their 8th choice on their rank list.
If you're open to the rest of the country, I would say 8-9 programs is the magic njmber (based on experience...great ITE scores, stellar recommendations, etc.). I interviewed in 5-6 Cali programs and 4 out of state programs. Matched #4.
Depends on where you are regionally and how many interviews you can afford to do from time/money standpoint. If you can easily drive to 8 programs, then apply to a lot. If you're like me, and don't have tons of time/money to interview, then I applied much more selectively. I applied to 8, interviewed at 4, matched at my #1. There were folks on the interview trail this year who were on their 8th or 9th interview. I just couldn't get that much time off or buy that many plane tickets and rental cars. Unlike interviewing for residency, it doesn't cost more money to apply to more programs, FWIW