Boris Gudenuf
Deity
It might be heavily dependent on map size if they follow the same one city per region approach of Endless Legend. So 10 Factions sounds already on the limit if that's the case. Each additional faction might require at least one additional region.
8 Factions in Endless Legend on a Huge map ran pretty slowly in the late game.
One presumes that they learn from their own previous games, or at least include applicable mechanisms that worked previously. As I remember, in Endless Legend you could set a relative size of region, so that if you wanted to stuff 'extra' Factions onto a map, you could set regions to Small and pack more room for cities onto the same map.
Something similar would be an easy 'fix' for either playing on a relatively empty map (set Regions to Large) or playing with 'extra' Modded or Expansion/DLC-provided Factions (set Regions to Small). You might not be able to build a Meda City in a small region, but at least there could easily be room made for enough cities to accommodate 12 or 15 Factions instead of 8 or 10 (whatever the Faction/Map Size combos they are planning - one presumes they will have Small, Standard, Large, Huge or some similar range of map sizes, with numbers of Factions to match)
It's an interesting point, though, just How Much Computer will be required to play with those gorgeous maps on a Huge array of tiles/regions and a large number of units, cities, districts, et al in the late game. As we've noticed, the city districts/Quarters have tiny people walking the streets, so there's a lot of \, presumably, CPU/GPU-hogging animation being added everytime you add a city or a 'working' tile to a city.
That could add up fast.