There's no way we can tell you what's the "normal" number of traps to use. Everyone has obligations, in one way or another. Weather it'd be school, a job or family. The amount of time you have is going to be the main thing, and it depends on what you're trapping too.
If you'd be in a good muskrat area, it wouldn't be out of the question to have a dozen or more traps set along 100 yards of creek. A dozen traps set for coyotes in a 100 yard area would be foolish, in my opinion. (Unless you'd have a super hot bait station, and a pile of coyotes.)
Above Ric was getting at this point, I'd rather have 20 or 25 good sets, properly tended and working, than I would 100 crappy sets, that may or may not work. Add in the weather, deer, livestock, skunks, possums, etc. It takes time.