Cheezy the Wiz
Socialist In A Hurry
I think you know this, but 31 = 11111 in binary, which is a bizarre breakup of memory.
255 is 11111111, which makes much more sense, since that is a full byte.
If the Civ coders wrote the cities into an array, then this limit for cities makes sense, and has a natural limit.
I am betting he can push beyond 31 civ's, but 255 cities might be impossible.
The city limit is 512, the "improvement & wonder" limit is 256.