Subject-by-subject contrasting essay (too many transitions?)?

I know when you write a subject-by-subject essay you need transitions in the second half of the paragraph, to show the reader that you're kind of.. changing gears I guess. Is there a rule of thumb about HOW MANY transitions are needed? Just one, or a few? Or can you have too many? I'd hate to put too few, but would be even more miserable if I put a lot of unnecessary words in my essay. Turning final draft in tonight, so I hope all of you English Major's out there can throw me a bone. ;)