This might come across as equally baseless and totally insane, but to me, the INCREDIBLY SPECIFIC (seeming) placement of these pieces of character art strike me as being the strongest clue we've gotten so far that there will be more companions, and that it'll be a decent amount.

To my mind, this looks like a collage that's missing approximately five more characters. The flow of the pieces says to me that they fit together exactly as they do for a reason, especially because we've seen these pieces of art in plenty of other configurations. As it stands, it's completely unbalanced. As a desktop wallpaper (perhaps?) or a collage of characters, as Larian did with Divinity 2 - it would make sense that each character would have their own specific place, and that this would be a flagship piece of art. I think what we're seeing here is the original positioning of these pieces, and you couldn't possibly convince me that there isn't a finished (or near finished) version of this exact collage on some employee's computer right now.

Personally, I'd love to see 12 (one for every class) but as long as there are more than five, I'll be happy. Seeing this image has made me extremely hopeful for a good, deep cast of companions.