Depends on body part and, as Swain said, the program.
Most of the time I do 10-14 for the larger muscle groups (legs, back), 9-12 for chest, then around 8 or 9 for bi's, tri's and shoulders.
It's crazy to go back and read Arnold's Encyclopedia where he's doing chest and back in the same workout - 35 sets each. That's the stuff which I first used for lifting. I did 20 sets for bi's and couldn't figure out why my arms wouldn't grow...