I would like to have NPCs for every class AND race, so 88. Oh, and both sexes. And nice and evil. So 352.
But on a more serious note, 8 seems like a reasonable number to start with. From what I've understood of the classes, they leave quite a lot of room for customization, so having a specific role being filled in a party shouldn't necessarily demand a specific class. And as for NPCs with strong opinions on certain playstyles, I trust Ziets and Avellone are able to either give them good enough reasons to stick with the party or have the more nitpicky NPCs fill a role that can be filled by some other NPC. And if one wants to have the one exact and correct party build then the adventurer's house is a reasonable tradeoff.
Having said all that, I would of course like to see NPCs for every class or even more, but one can't have everything. Quality > quantity and all that. Luckily it's almost guaranteed that modders will take care of any glaring deficiencies in the NPC roster quite quickly. Maybe not at the level that Avellone or Zietz could achieve, but if they don't have enough time to do their job well, we won't get that level of quality for the 8 either.