Larian has not confirmed the amount of companions.

I think the safe bet is to say that we will be getting 8 companions given what we know. If you are an optimist I think you could hope for 12 - one for each class and we have just seen nothing on four of them.

It also would not shock me if Larian eventually announces that there will be 'origin companions' and also 'sidekicks' like in PoE2. Sidekicks being party members you can play with - but that do not have the depth or major story of the origin companions. I think good examples of characters who could become 'sidekicks' are Halsin, Minthara and Gandrel.