Europe has 44 nations (according to the UN), which includes city-states and small countries like Lichtenstein, Andorra, Monaco, Vatican City, San Marino, Malta, and Luxembourg.
Wanna make a guess how many countries, territories, and city-states that Faerûn has? You are welcome to count them all up yourself, but I have more productive things to do with my time.
Someone (you?) said there were 27 realms or something, I was using that as a comparison.
Sure, it depends on how fine-tuned you want the various write-ups to be, but I would assume that fans of FR would want sufficiently useful write-ups so as to be utilitarian for actually running a game in the setting plus some other DM and/or player-side goodies and a sample adventure included.
Depends what "sufficiently useful" consists of, I suppose.
Compare the late-1e-era boxed set with the 3e-era book. The box-set has a map with lots of blank space, and books with write-ups in just enough detail to give an idea of what goes where; with the rest being left to each individual DM to fill in as needed.
The 3e-era book's already got more detail than it needs and fills in far too much blank space on the maps.
In-depth town-by-town details are complete overkill.
So, what does it need by rough page count?
First, fold-out maps - plural - detached from the book itself. One of the whole thing, a zoom-in of the Sword Coast area, a zoom-in of the Shadowdale area, and maybe a city map of Waterdeep.
A general overview and introduction to the setting - 10 pages (all page counts include art)
Some details as to various guilds and factions within the Realms (Harpers, Red Wizards, etc.) - 10 pages.
A couple of somewhat-detailed write-ups of realms, as samples for DMs to follow if they like (maybe Thay and Shadowdale?) - 20 pages each, so 40 total.
A sample adventure - 10 pages.
We're up to 70 pages in the book thus far; leaving 250 for everything else assuming a 320-count total is the goal. At an average of 5 pages each that's 50 write-ups you could fit in there, be they of cities, realms, or whatever; a couple could be dropped in favour of short one-page write-ups of up to ten key NPCs.
Note that a separate map of each element being written up - other than the sample two - does NOT appear in the book; merely a grid reference to where it can be found on the fold-out map(s).
What did I miss?