State of residency is key. Applicants from California tend to apply to far more schools than, say, applicants from Louisiana, because of the vast difference in instate competition for their respective state schools.
Relative competitiveness of your key stats is also, umm, key. (note to self: use a thesaurus next time...) Where do you stand with MCAT and GPA relative to the median matriculant stats which, IIRC, are around 3.7/31...
IIRC consider that the average applicant applies to 13 or 14 schools - this is a stat that is available on AAMC - so keep this number in mind, but don't be a slave to this number, either.
IMO, an applicant with median matric stats, from most states, can be successful applying to 10 schools, so long as those schools are not all in the USNEWS Top 10 to 15...the same applicant from California probably needs to apply to 15+ schools. Applicants with subpar stats probably need to apply to 20+ schools. For any applicant, there is a point of diminishing returns, probably at 30+ schools...management of the process, mainly in staying on top of secondaries, will come into play with 30+ apps...not to mention the cost.
My rule of thumb for ALL applicants, regardless their stats: Apply to at least 3 schools that are not ranked by USNEWS; if you are lucky, and you have state school(s) that meet this condition, you can pretty much apply to as many top ranked schools as you like without worrying about the typical recommendation to apply broadly...
Full disclosure: I have above median stats, and come from a state with at least one school that is not ranked...I have designated 25 schools for the primary...I think it is overkill, but I did it thinking that I will get screened for at least a few of the secondaries - my plan is to do 15 or so secondaries, so I will be editing my secondary list in the months to come...hope for 8 or so interview invites...if I am so lucky as to get an acceptance at one of my rolling state schools in say October, the list of schools that I will continue to attend interviews at will be subject to severe revision downward...once I get an acceptance, the whole game shifts, and I will be extremely selective about additional interviews and cost.
