Some schools require 2 science professors, so I would recommend getting at least 2 science, one dentist, and one random (unless you do a committee letter). I majored in something non-science and had my advisor write a letter, so I ended up scrambling for another science LOE. I had to send it directly to few schools.
Why not look at the ADEA book, pick out some schools you like, and look at their LOE requirements - and try to cover as many schools as possible?