Here are some good criteria to follow:
Location: An excellent way to trim down your list of schools is to look at location. Rural or urban? Hot, temperate, or cold climate? Be honest with yourself and make a decision. If you want to live in a city with a warmer climate (e.g., Dallas or Atlanta), then only apply to medical schools in this environment. Don't make any exceptions.
Curriculum: Another good criterion for selecting med schools is the curriculum at schools. There are significant differences among the 125 med schools in the U.S. For example, some med schools REQUIRE their med students to do basic science or clinical research (e.g., Duke allots an entire freakin year to mandatory research). If you have no interest whatsoever in research, then do not apply to a school with a research requirement. Some medical schools have very traditional curricula, involving 2 years of lecture-based preclinical courses and 2 years of clinical rotations. Do you want a clear separation between preclinical (i.e., nose in the books) and clinical work (i.e., interaction with actual patients)? If you do, opt for a traditional curriculum. If you do not, then you should apply to med schools with longitudinal integration of preclinical and clinical work, meaning that you see patients in the 1st year.
Some schools have deviated considerably from the traditional curriculum by exclusive implementation of problem-based learning (e.g., Harvard). Others mix it up--the lectures are supplemented by PBL (e.g., Baylor). So, curriculum is very important. Research it well.
Price tag and prestige: A third criterion is the price tag. If you're interested in academic medicine, prestige is important, but not so important that it overrides location, preferences regarding the curriculum and, most importantly, your career aspirations. An important thing to keep in mind is that, generally speaking, prestige translates into tremendous debt (the average indebtedness of graduates from highly prestigious med schools exceeds $100,000). There are, of course, exceptions to this correlation (UCSF, Baylor, and Mayo come to mind). Nonetheless, if you are 100% convinced that academic medicine is not for you, meaning that you hate research and teaching, then prestige should not factor into your decision. If private practice is your goal, then cheaper is wiser. Go to the cheapest med school you can. If you cannot positively rule out academic medicine at this point but you're leaning towards private practice, then you should apply to a few prestigious programs (one or two from the top ten and 1-2 from the top 20, according to USNews rankings).
Credentials of previous matriculants: It's important to look at the credentials of previous matriculants. To have a decent shot at a given school, your numbers should be comparable to those of former matriculants. Now, this does NOT mean that you are absolutely barred from acceptance to a given medical school if your numbers are considerably lower than the published average. People get into Harvard with a 24 on the MCAT. Numbers are not everything in the admissions process. Indeed, you ought to apply to ANY medical school that seems like a great fit for your temperament, preferences, etc. However, comparable numbers generally translate into a higher probability of being invited to interview and, hence, a greater likelihood of being accepted. Just something to keep in mind when you decide on which schools to apply to. Remember: each medical school will probably set you back $100 or more in application fees. It's OK to apply to dream schools; just do it wisely. Don't throw away your money.
In terms of the actual number of schools, I think that 12-15 is a good range; outstanding applicants can lean towards 12, less than outstanding toward 15. If location, curriculum, price tag, prestige, and credentials of previous matriculants are seriously taken into consideration, then 12-15 should be just right.
