If you look at my old home of Hamburg:
HSV and St Pauli are obviously full-time (HSV being historically way bigger). Altona and Norderstedt who play in or just outside the city are not and probably never will be, there is not room for three or more professional clubs.
Hamburg has a population of 2,000,000 or so not counting the surrounding areas. Officially they say it's 5,000,000 for the Metropolitan area but that includes bits that I'd say have nothing to do with Hamburg.
Berlin (Hertha and Union) and Munich (Bayern and 1860), same deal.