Right now the game doesn't have a formation menu, but usually with other rpgm games you are limited to four active members and any other further companion is added to a "reserve" list that you can swap in at will (as Eva has mentioned above). I'm not sure if this can be circumvented with plugins (fairly possible).
I mean, yeah there's plenty of characters that could potentially be added to the possible list of companions, but tbh I doubt Chyos will implement them all, or even that many to begin with.
Obviously this is something only he can confirm but going by what Chyos has commented (like how the bandit camp fire being cluttered already and how that will be an issue when Mira and Raven gets added in) as well as all the issues whenever someone gets added in (story/events/defeat scenes reworks needed for every single one of them), we have to factor in the feasibility of the work needed for it.