You should request LoRs only from faculty who personally know you very well, either through your taking courses under this professor, conducting research under his/her guidance, etc. LoRs from non-STEM faculty members are perfectly fine. These can serve to highlight your academic abilities outside of the sciences – an increasingly important aspect of MD/DO admissions. I had five professorial LoRs, four of which were were from STEM faculty, and one of which was from the professor I took for freshman English Composition I & II, and whom I also had for a World Literature course. In my view, I would recommend (if possible), five faculty LoRs, at least three of which should be from STEM faculty and no less than one of which should be from non-STEM faculty.