What is likely to matter most, is that the party composition ( including pets ) and the choices you go through to select them are intimately connected to the development of the characters and the story lines in a satisfying way.
In the Dragonage games, for example, you could certainly gather together a three-ring circus if you chose, but there were certain characters that you could not dispense with, however hard you tried; that was not necessarily ideal either, but was driven by story imperatives.
Personally, I am not fixated on number or choice of companions as being more important than the story; but having said that, there is more than a suspicion that the 4-character co-op emphasis of the game is fixing these choices more than the nature of the tale. That might suck for SP players.
I'd actually be surprised if there were many more origin characters just due to space constraints on the Nautiloid. There's no law requiring that all party members be tadpoled, mind, so not every part member has to be an origin character.
There have been no absolute promises of any more fully-acted characters, to the best of my knowledge, although Swen has suggested there will "probably" be more. I suspect they already know exactly how many companions they expect to have, as it would be difficult ( though not impossible ) to add characters late in development, given the depth of interaction.
If you look at the Nautliloid in the intro, you see the Illithid enter a room of containment pods. There appear to be 9 pods in the circle, at least some of which are empty, with only Lae'Zel and the viewer ( you ) definitely present in the pods. That would leave possibly 7 free pods, which, by coincidence, appears to be the number of people in Yartar that are dematerialized and transferred to pods.
None of them appears to be the other origin characters; that could either mean the additional origin characters were unknown to the company making the intro, or it could mean that the Nautiloid has multiple containment rooms, so it is not necessarily that helpful.
But if there is a pre-planned number of origin companions, and I were to guess, then it would be 8/9 ( depending on whether the viewer in the intro is one of the companions ), to fill out the chamber we see on the Nautiloid.
