How to add a faction

Sorry if this is in the wrong place.

How do you add a new faction? Ive had a look in the forums and cant find any tutorial. Also what is the limit on new factions (The last question is just my general interest since the last MTW could have a fair few new factions).

Well there are these in the modding tutorial forum:

Duke of Serbia's New Faction from nothing:
https://forums.totalwar.org/vb/showthread.php?t=77806

New Faction using Normans/Saxons:
https://forums.totalwar.org/vb/showthread.php?t=76730