I don't have any source on this, but I've seen others saying that Larian has stated that there will be mercenaries that you can hire on as party members. It sounds like they will be customizable. To what extent I can't say, as I haven't been able to find where Larian said it.
I hope the mercenary system will basically be the ability to build additional characters, but I guess we will see.
I think the question was about party capacity, not about additional characters with different classes available than what we have.
