This question came up in conversation last week. I had guessed 1,000, while the others I was talking to suggested 400-500. As I was stuck here at home for the better part of Monday, I tried to figure it out for myself, using the net as a resource.
After doing some web searches and evaluating a few relevant websites for accuracy, I found the Verizon Yellow Pages site to be my best source. Unfortunately, the only way to get a count was to copy all the restaurant listings for San Antonio from the site and paste them onto a spreadsheet for analysis. This turned out to be 2865 listings. (The Mexican Restaurant category on the site was useless.)
Once I got everything on the spreadsheet, I sorted and deleted all the non-essential data, and then flagged all what I determined (by name) to be Mexican Restaurants. Then I rechecked the questionable listings a few times, and came to a final number of.....907...!
This was based only on restaurants with San Antonio as the city name listed. And I didn't include any Taco Bells, just because.
Assuming my total is at least close to correct, I have to believe this count is the highest in any city in the country for a specific cuisine.