12,14,16,24,32 as the map sizes increase
Added following to answer rest of your question:
Is it worth it to build more? Depends on your goal. Pretty much required to win via domination. Can help with culture wins, but becuase of very slow/costly builds the additional cities won't help much. Can help with conquest by denying the land for other countries to build on. If trying for high scores, it is pretty much required as well, as the additional territory and workers adds to the score. Doesn't help much with SS or diplomatic victories.