It really varies with some schools. I was surpirsed that Jefferson wanted one each from a bio, physics, chemistry and humanities professor (unless you have a pre-med committee, which I do not). Unless you are prepared for schools like this, you can get caught in a bind. I had to track down my physics TA from freshman year to get him to write a letter that the prof will co-sign (probably won't be the strongest letter, but I have to go with it!)