I just think it should be left up to the player to decide. The game should not limit you for no good reason.
By that logic the game would allow player to run with every companion and mercenery possible in the game. Even games where you manage a whole army have some kind of cap - be it a "supply limit" or "upkeep" cost. A hard cap of "you can have X number of characters" might be a crude limitation, but I don't think a game like team-based RPG need a more nuanced system. You pick 4/5/6/8 characters for your team, and that's it. Easy for player to understand, predictable for devs to design around.
