I applied to 24 schools, which I feel was the right number for me (3.75 GPA, 37 MCAT, CA resident) - 11 IIs, 2 acceptances (0 acceptances at state schools).
I think this depends on 1.) where you are in terms of MCAT/GPA/ECs and overall competitiveness, 2.) state residency.
There are 141 allopathic schools in the US.
For those of us who are not competitive, you can just ignore the Ivies/top 20-30 schools (Harvard, Yale, NYU, Penn, UCSF, Wash U, Columbia). Conversely, if you are very competitive it's probably not worth your while to apply to lower stat schools that get tons of applications (RFU, GWU, NYMC, nothing against these schools), which might be about 10-15.
Since you can only have residency in one state, if you are OOS, you can also ignore the schools that are extremely state specific (state schools in CA, TX, NJ, FL, WA) - I counted about 50, counting public schools with <20% OOS
from this table. With a few exceptions, even the schools that have >20% OOS are still a gamble and may demand stats far higher than those of IS. I would add about 10 where this appears to be the case (OHSU, UH, UNC).
Unless you are URM or have ties to Puerto Rico, there are about 6 historically black/Puerto Rican schools you can cross off your list.
You can probably apply to fewer if you are from TX or FL, which have a lot of medical schools and restrictions on the number of OOS students who can be admitted.
141-30-60-6 = 45 schools.
(There are, of course, others such as schools in areas where you really don't want to live or state schools that give you sticker shock at their high tuition, if they even give you an interview as an OOSer.)