Well, there really aren’t too many “cities” so to speak.
At least not “public cities”
by which I mean, open/free cities that are large. The Courts is the largest city, since that’s where the Cofactors, Triumverate and Supreme Septat hold court.
In fact the “Courts” are multiple courts
The Court of Co-Factors
The Court of the Triumvirate
The Combined Courts of the Quintenal Cabal (a joint session of the Co-Factors and the Triumvirate)
The Synod of the Supreme Septat
So it’s big, huge.
There are a few other smaller towns/cities that are public/neutral ground. None are huge, maybe a few thousand demons in the largest. And a good number of these, while technically “public” are actually groups of loosely aligned demons who group together for protection. So while “public” they aren’t necessarily friendly to outsiders.
What other “cities” there are, are really enclaves/villas/domains/etc of various Demon Lords and Princes. These are their “private” cities in which they and their followers/henchdemons etc live. I.e. this is where they bunk their demonic hordes, their armies, etc.
These can, however, be quite large. However, no one really officially “knows” how large any are, since this is considered sensitive information.