Around our parts here, it depends on what level and whether it's 8 or 11-man ball, so here goes:

11-man varsity: 5-man crew
11-man JV: 4-man crew
11-man reserves: 3 or 4-man crew (depending on how anal the schools are about paying for the crew)
11-man freshman: tend to be 3-man crew, unless you have my crew cheif, who's a retired salesman and can convince the schools of the need for a 4-man crew!
11-man JH: 3-man crew. That's for tackle football. One public school system with about 10 junior high schools don't do tackle but flag, so they use 2-man crews for that.

8-man varsity: 4-man crew
8-man JV: 3 or 4-man crew (depending on the school)
8-man JH: 3-man crew

The state playoffs in Nebraska use 5-man crew, as does Iowa.