Several years ago I believe it was USA Today that ran a story about cities on a per-capita basis which had the most chain restaurants and if memory serves correctly it was Columbus, Ohio that was the "winner", however both Denver and Indianapolis were indeed in the top 5. Buffalo, NY was up there too I can recall( though being a dying city as it is, I have some doubts on that pick ) and interesting Richmond, Virginia made the list too. If this list only includes chicken fast food and those all you can eat buffets then Richmond could quite easily be tops. Back in the early 90s when I was there on business it was rather shocking to drive on Broad Street ( their main street ) and see all those chicken and buffet places, which it seems to be located on every other block.
Somewhat off topic but a friend of mine a few years back relocated to Indianapolis and I remember her telling me once about how she was amazed at the number of supermarkets that city had. Not just multiple locations of the same chain but rather the number of chains that serve that area. I believe she said nine or ten different chains had stores there. If that is true then that is very unusual since many larger metro areas such as Baltimore-Washington, Philadelphia, and I believe even New York City are served only by at least 4 or 5 different chains maybe 6 tops but not ten.
On the flip side of this, the largest city with the smallest per-captia number of fast food I would guess has to be San Francisco. Isnt this the same city that doesnt allow big box stores? If so I wouldnt be surprised if they had a limit on fast food places as well.