Datamining, Larian's statements. All companions are going to be origins characters, and there are only eight origins characters. Aside from the five we have, there's
Minsc, Helia, and Karlach
. Which itself is deceptively large, because unless Larian changes their plans, we'll be losing a minimum of five of them at the end of Act I ala DOS2.
Depressing if this turns out to be true but not really surprising at all, given that every interaction is cinematic. A poor, poor decision to insist on Origins characters only and a pitiful number of companions at that.