I think that 8 would be enough if they were diverse enough in terms of race, gender, background, outlook and party role.
I mean, whatever type of playthrough I was going for (good, evil, lawful, chaotic, polite, rude, violent, pacifist) I'd want there to be some companions who I could travel with who it'd make sense for me to travel with. It seems like there'd be an obvious temptation to make most of them human, male, basically good, etc. with a couple of token "others" thrown into the mix. What I'm saying is if one of my companions isn't a homicidal dwarf lesbian, I'll be very upset.
Also, in terms of party role, whilst it'd be nice for them to have a clear role; I wouldn't want it to be so limitting that it affected my decision to get them. I wouldn't want to have to turn down a character who seemed cool just because I don't need another melee fighter. So they'd need to be diverse in what they could bring to the party gameplay-wise, but not be limited to the one task or the only ones that could do that task well.
Like, most of this is obvious, I think. But I think one of the reasons why people are debating this. It's easier to fall into these pitfalls the less amount of potential characters you have, and adding more would be a safety net. So... that's my tuppence-worth, I guess. Eight will be enough, providing certain obvious, yet dangerous, pitfalls are avoided. More would be nice too I guess, but not too many. This isn't pokemon.