I think it'll likely be either 4 or 5, because of the way challenge rating is calculated in 5e. It assumes that you have 4 players of the same level of the challgenge rating of the listed enemy.
But if they allow a companion of each class it'll total 12, so you'll probably be able to "store" the ones you are not using and switch your companions whenever you want.
Why make a major change to the gameplay everyone is used to instead of simply increasing the "challenge rating" to account for 6?
