The legality depends on locally applicable zoning laws, sanitation/environmental laws, and probably a few other laws. The answer would likely vary from place to place (and sometimes also depend on how many sites are involved), and possibly on whether this is done commercially for income or not, etc.
In Vermont, the environmental agency requirements (sanitation and potable water and such) recognize four or more sites as a "campground" with specific requirements, while three or fewer are just camp sites and have much less regulation. That is in addition to any applicable town zoning regulations.
