Parmandur
Book-Friend, he/him
So, Light Of Xiraxis (sp?) has 10 chapters, going from page 8-64 (some intro material up front), for about 5-6 pages per episode.To follow up on my previous post, the chapter on Sharn in the 5e Eberron book was 32 pages long, and the description of Waterdeep in W: DH was 27 pages, which seem to be the best comparisons for large cities in a 5e product.
Also, if the Factions are in, it would be reasonable to assume that they will get at least one page each.
And, beyond all that, there could be some overlap with the adventure book. If the adventure mainly takes place in Sigil (which seems logical), that means that locations from the adventure won't need be to covered in the setting book. This would be the case for other locations as well, such as portal towns...
And finally, I assume they will have a poster map, which better be double-sided, with Sigil on one side, and the Outer Planes on the other!
That means that this Planescape Adventure could have as many as 18 chapters...enough to get a slice of each Plane.