If you send your letters through a letter service, the schools will have no choice but to accept what is sent in the packet.
6 letters is great, same number that I have. Just try to make sure they are all offering a different perspective your indiviuality. Do not send 6 letters from your teachers, it's monotonous and unecessary. But I do suggest varying the letters, a couple science, a non-science, volunteer supervisor, doctor you shadowed, researcher, etc...
Good luck, but luck isn't necessary, because we have each other.
peace
H