There is no such thing as a "ducal seat". Any city, townsland, or stronghold who's lord is not already a duke, can be made into a new duchy by the ruler, with the lord of that region becoming a duke.
You can have an unlimited number of duchies in your realm. Every noble could be a duke. You can have extra, empty duchies left over without dukes.
Whats the point of requiring the starting region to be a city, townsland, or stronghold?
