Well, the Sagittarius from Vanguard/Seekers has a single doctor and a medical technician (and another character in another department has an M.D. and a few others are trained medics) for a crew of 14.
In terms of fiction, my own fanfiction starship has a CMO, a head nurse, two doctors-in-training (residents), two junior nurses, a counselor, a Chief Corpsman with battlefield experience, a pharmacist and medical lab technician (both Corpsmen 1st Class), and an EMH (making the total 10/11) for a crew of 200+. However, now that I'm thinking about it, I'm not sure I need the two Corpsmen on this ship, and could move them to the ship's home port and making them part of the base's or sector's medical staff.