Huntsville AL has a population of about 150,000 "in the official city limits" about about 225,000 in the "metro area (15 minutes drive in bedroom communities outside town).
At one time we had 5 dive shops in this area. so 1 per 30,000 city and 1 per 45,000 metro. That's a lot of people per dive shop, especially considering we are landlocked and a 6 hour drive from the nearest ocean/gulf. Ironically they have all gone out of business except for one in the past 5-8 years.
However, in a neighboring city 30 minutes away with a population of 50k, another dive shop has opened about 5-6 years ago and have been giving them all a "run for the money"
Birmingham AL has several shops also, but they seem to be dwindling also.