For a few play reasons. First is character saturation. If we have too many characters at the start it can get really confusing and it'll already be a tangled mess. Naturally starting with 1, and then when people get more comfortable, we can start adding more, is best.

Next is whether or not the players can play those two characters sufficiently. This comes down to players, but some people don't know whether or not they've bitten off more than they can chew.

I think it'd be fine adding more characters, but please cut some off if they're not necessary or don't add to the current cast properly.

I mean I know it's an RP, but there's a reason that shows introduce characters bit by bit. Best we can do is start with one character per player, and start including more when relationships are more established between both players and characters.