As confirmed in the in the Q&A of Community Update 6:
Modding will be supported, but not before 1.0, not during EA. Again, we really need to focus on working with feedback and creating the game.
Early Access will allow you to control a main character with 3 companions. This will not change during the entirety of Early Access. (Approximately a year)
Mod support will be worked on after the game is officially released sometime in late 2021. At that point someone will likely create a Mod to support the control of more characters than the game is balanced for.
If the control of 6 characters is a deal breaker for you then I would avoid playing Early Access.