I commonly play with 28-32 factions depending on the build that's released (they all bug out after so many custom factions

yes, if I remember right it's like in rows of 5 going down, 5-10-15-20-25-28 or so (& when you make a bunch of custom factions to include in the game be sure to save your game at 'turn 1' so you can replay it or else all the customs factions turn to 'neutral' intead of being able to join in as kingdom or empire the next time you try to start a game using these same custom factions) hope this makes since

edit: in the faction creater

Huh I knew you could add all the starting factions and then 1 custom one but I thought that was it.

ya would like to have like 50 major factions with 3 minor faction per each major, this with the wildlands would make for a truely epic game (Would really be nice to have a detailed race builder that allows for different body styles & such, heck would be like a d&d realm

Edit; ally with all three minor factions around you & it turns your lil kingdom into a massive alliance allowing a battalion or two of elves, lizardmen, & minataurs to join your ranks (though I would prefer for the minor factions to remain ai controlled  except for the units purchased from them

I favor 6 to 8 well thought out and developed fractions than a mish-mash of 28+ cookie cutter major fractions with no personality using the same AI routines.

If I could have 100s of factions like those in the ExpandedFactions mod, I'd play with them all.

In 20 years when we all have massive computers this game will still be playable. There is no limit to map size and no limit to number of factions. Just think of how large the game could get. 

Ha! Probably.