Well, generally, I would say 2 outfits per day.
But if our destination is somewhere really, really hot, (and sweaty), I must remember to pack waaay more underwear so that we don't get stuck wearing dh's undies like we did on one trip we took, lol.
He may only bring 3 outfits for a 6-day trip, but he seems to have the foresight to load up his suitcase with tons of underwear!