Its pretty difficult to say how many you should apply to. There are so many other factors besides USMLE. It also depends on what part of the country you are looking at, etc, etc. I would say roughly, if you have <215 (I picked this number, because I have heard some schools use this as a cutoff) on Step I, apply to 20 or so programs, and you should land enough interviews to match. That's not saying if you have a 216 and you are applying to all the Cali programs + Columbia, Cornell, MGH, BWH, BID, JHH, Duke, Mayo etc that 20 will be enough. Just make sure you apply to a good mix in terms of prestige and location.
Like powermd said don't neglect the transitional/prelim IM or surg apps. I only applied transitional and sent out 4 apps, interviewing 2 places, which were both fairly competive in terms of lifestyle. I was lucky enough to scramble into a prelim IM spot at the school I matched for gas, which so far has been really nice. But it is really easy to screw yourself if you don't pay attention to it, ie you may get stuck with a prelim surgery spot or malignant IM position.