My advisor always touted having 4 letters,2 sci and 2 non-sci. I am applying this year and have found certian schools have odd requests (requirements). Jefferson wants a letter from a chemistry, physics and biology profs. NYMC wants rec. from your major dept. But mostly amy four letters have served me well. I double majored in Biology and Art so I have recs from profs. in both dept whom I worked with and know me well. I also have one from an anthopology prof. who I took alot of classes from. The key is to get your requests to the prof. early, perferably when they still remember you. 😀
Letters from Volunteering are optional at some schools, it can never hurt to collect as many good recs. as you can.