Character Regulations
You are encouraged to create as many characters as you need. Mostly, these characters should be part of your own faction, but there is no hard rule preventing you from imagining characters with different allegiances if it allows you to better tell your stories. That being said, you are strongly encouraged to reach out to other factions before creating characters that "belong" to them.
There are no limits on the number of units any single player may command in the game but they may only command units for a single faction. In simple terms, you can't belong to two factions and command units for each of them.