My school puts all of our LOR's into a single letter, if it's from our school. So all my schools will be 1, I can get another from the MD I'm shadowing, one from my Pharmacist that I work with.
So as for mvenus' post, how would that work? I can get a professor from my major to write one but it'll just go into the single letter LOR. The letter from my PI will go into that single letter LOR as well (he taught me before I worked with him).
As for non-science, maybe someone I volunteered with.
--edit
To follow up on that combination LOR, I have:
At least 3 professors from school, all science
1 non-science professor
1 MD/PhD
DEAN OF ADMISSIONS
writing me LORs.
And it will all just get combined into one generalized letter. I am going to fight bone and teeth for Dean's LOR to be separate.