Theoretically, if a city was made up entirely of flood plains, irrigated, and railroaded (21*5=105 food) this would be 52-53 citizens. Add in say 3 wheat and you get 55-56. Possible to have more I suppose, with more wheat squares, but that city will only produce 1 shield/turn! Realistically, most people see numbers in the 35-45 range as their highest with normal terrain.