Filming a group of people interacting is harder than filming two or three. Also, consider possible transport hassles, accommodation, etc. Avoid too many locations. It takes time to shift gear and people, and sometimes costs money if you have to drive from one spot to another. One guideline suggests you figure one location per day of shooting, but that depends a lot on how much you film at each location, and how hard the scenes are to film.
Public places are a hassle.
Crowded public places are even more of a hassle. Street scenes mean getting permission from some local government agency to close off a section of the street or sidewalk, having people manning the barriers, etc. Public interiors are no better. Restaurant owners will not appreciate you wanting to shut down their restaurant to make your budget movie, and they don't want their customers irritated by a bunch of cowboy film-makers either. Also note that anyone who appears in your film needs to sign a contract giving their permission to appear, so thinking you'll just include all of the passers-by isn't the answer either.