How competitive of an applicant you are
Are a US senior or IMG
How far you (and spouse) are willing to move
How willing you are to go to a less desirable program (less desirable defined by you). e.g if you rank 32 programs and match at number 32 are you going to be okay with going there?
How much money you have to spend on sending out ERAS apps and going to interviews.
There is no magic number, it is different for everyone. Generally speaking, it is a numbers game. The more apps you send out the more interviews you get. The more interviews you go to and places you rank the more likely you will match.
Check out the NRMP data for PMR for 2011 to get an idea of how you stack up with others that matched in PMR: http://www.nrmp.org/data/index.html (charting outcomes in the Match on the bottom right hand side).
I hear the "magic" number of interviews and rankings to match is 8-10. I would apply to as many program as possible and cancel interviews if needed. Ultimately you get one shot at this and by going to multiple places to see if you will fit in at that program is worth the time and effort. Good luck.
apply to many, interview >5 but more than 12 and you're not really increasing your chances by much and spending a lot of money. plus turning down interviews actually feels good, inflates your self worth