OCN = Optimal City Number
From the editor: Determines the optimal number of cities before rampant corruption occurs. If a player exceeds this number of cities, corruption dramatically increases, regardless of how far away from the capital a city is.
Basically, it is the number of cities you can have until corruption gets really bad. It is affected by world size and difficulty level.
OCN #s:
Tiny - 14
Small - 17
Standard - 20
Large - 28
Huge - 36
(you need half of the OCN to be able to build the Forbidden Palace - so 7 on a tiny map)
Multiply those numbers by the difficulty level:
Chieftain - 100%
Warlord - 95%
Regent - 90%
Monarch - 85%
Emperor - 80%
Demigod - 70%
Deity - 60%
Sid - 50%
That means, on Sid, on a standard size map, you only get 10 (20 * 50% (or .5)) cities before corruption will drastically increase.