Atlanta during the BBS era must have been paradise. It had the largest local free calling zone of anywhere in the US. Where I lived, the town on the other side of the interstate was in-state long distance, so we were confined to the BBSes in our little suburb plus the nearby "big city" of about 100k people.
Maybe some places like the Bay Area and New York City had more local BBSes but it wouldn't surprise me if Atlanta was in the top five or even the top given the huge free calling zone.