8 lors for md/phd is normal, but not for md.
my premed committee suggested 2 science profs, 1 humanities, 1 research, and if taken time off, 1 from that activity as well. unlike dianamd, my premed committee sends every letter in applicants' files and refuses to pick the best ones. there are pros and cons to this, though.
since you are a humanities major, i would definitely recommend having at least 2 lors from science profs. it seems that you have lots of time before you need to worry about gathering recommendations, so in the mean time i would try to form relationships with all of your science profs. there is nothing wrong with a TA writing a rec for you, but you'll be at an advantage if you have profs write them for you, esp if they are well known in their field. of course only if they know you will this be an advantage.