Don't build too many cities...
Seriously, a limit of 255 cities is hard-coded into Civ2. There is a fixed-size table in the data file that keeps city number, name, size, improvements, and other items for each one. Given that you need to have at least one city belonging to another civ, your own limit for building cities is 254.
Personally, I find managing more than 50 or so cities an unbearable chore, but then I am a perfectionist micro-manager...