Let's do some basic math for my ACKS: The Dark Project.
The City is the largest urban settlement in the campaign area. In terms of realm size, The City and its rival city-states are Principalities, with 18-65 or so 24-mile hexes each. The City is ruled by a Prince (a title which fits the ACKS domain system better than a lowly Baron, IMHO) and so are its rivals.
The City has a realm of 47 24-mile hexes, a sizeable Principality; its main rival, Blackbrook, has 44 hexes; Cyric has 35 hexes; and Bohn has 27 hexes. Everything else on the map is no man's land, either badlands between the city-states' realms or wilderness into which the forces of Chaos creep.
The Thief world is a relatively advanced one, technologically speaking, but also a world well past its age of Imperial glory. So I'd use the default average population density, especially since not all hexes in a realm are proper farmland - most realms have many hexes of mountains and woods, which, while providing rich resources (such as timber, metal and coal) in many cases, have a much smaller population than typical farmland. So, on average, it's 50 people per square mile, 300 families per 6-mile hex and 5,000 families per 24-mile hex.
Also note that most of these people do not live in the countryside but rather in the city-states themselves.
So the base population sizes would be:
The City - 250,000 families, 1,250,000 people total (I bumped this to 250,000 because this is the most developed realm in the setting)
Blackbrook - 220,000 families, 1,100,000 people total
Cyric - 175,000 families, 875,000 people total
Bohn - 135,000 families, 675,000 people total
However, urban demographics greatly vary from the ACKS norm, as this is a world of city-states, and advanced ones at that. So The City and Blackbrook realms go 2 rows downward on the Urban Population table AND 2 rows downward on the Largest Settlement table. The other two cities go 1 row downward on the Urban Population table and 1 row downward on the Largest Settlement table
So how does the urban population look like (in families)?
The City - 250,000 families total, 125,000 urban families total, 62,500 of whom living in the largest settlement (The City) which is a Metropolis and a Class I market.
Blackbrook - 220,000 families total, 50,000 urban families total, 30,000 of them live in the largest settlement (Blackbrook) which is a Metropolis and a Class I market.
Cyric - 175,000 families total, 35,000 urban families total, 12,000 of them live in the largest city (Cyric) which is a Large City and a Class II market.
Bohn - 135,000 families total, 27,000 urban families total, 10,000 of
them live in the largest city (Bohn) which is a Large City and a Class
Yes, The City is HUGE.
Now, on to realm subdivision into Duchies:
The City is a Principality. The Prince himself rules only The City directly - it is wealthier than any Duchy under his command and enough of a hassle to manage all by itself. All his lands outside The City are divided between the five Dukes.
Blackbrook also has five Duchies.
Cyric has four Duchies.
And Bohn has three duchies.
Generally speaking, each Duchy is dominated by a City of about 3,000-4,000 families, a Class III market.
So here's an updated work-in-progress Campaign Map: