The Boston Zoning Commission has set a limit of four on the number of college students who can live together off campus. That could spur a citywide crackdown on crowded student housing. Some students and property owners condemned the limit. They argue that it further burdens cash-strapped students and may force many into more expensive college residence halls. But proponents--a coalition of neighborhood groups, college officials and city leaders--contend that the new occupancy limit will reduce the number of rowdy late-night parties on otherwise quiet residential streets.