I have nine:
1 each from Biology/Physiology, General Chemistry, Organic Chemistry, and Physics.
1 from Biopsychology (I had her for two classes (Biopsych and Research Methods), and I later TA'ed her Biopsych class).
1 from a clinician I worked with and who ran the code that got me interested in medicine (my first time restarting a heart!

).
1 each from my dissertation committee members (a theologian, a philosopher, and a psychiatrist (with whom I had also worked clinically)).
My pre-health committee wanted at least five, and suggested hitting the core sciences.
EDIT:
All of these went to my pre-health committee first, but will be sent along with the comprehensive pre-health letter.