We already know at least 3 other origin companions will be coming because of what Larian has said. (And there is datamining indicating who they could be, though that might change). Additionally they said they started us with the companions that would be the most contentious (leaning toward neutral and evil) so they could get feedback on them, indicating that there will be good aligned companions coming.
The graphic you linked is to advertise and show off the early access as that is the cast at the start of EA, but it will surely increase at least by three, and I hope more. Also basing the party size off of the cover isn't exactly the most accurate thing as covers can be pretty misleading in my experience, especially with party based games cause there are either less or more characters on the cover than I can actually control in my party.
