I think the leaked Seven could be fine. Mass Effect had a pretty strong cast, everyone had a specific place in the team and they were all varied enough to stand out, regardless of the fact that many people didn't like some of them (paticularly Ashley and Liara it seems. I know that many people find Kaidan boring, also). But, at the same time, I feel less positive about the limited number in ME3, even though technically it was one Prothean bigger than the cast of 1. For starters, almost all of the companions were returning characters, and I feel that as such most of them had already ran their course, and there wasn't really the same feeling of getting to know them as there had been in the other two games. While Garrus, Tali and, to a lesser extent, Kaidan are among my favourite characters, I feel in this instance they didn't add as much as they could have, and less than a new squadmate could've. Secondly, I played through the game without, in my opinion, the best squadmates in my first playthrough due to not having the DLC at the time. And then, besides Javik who wasn't exactly one of the core characters of the game, the only other new character was James, who I personally cared little for.
I know this judgment is somewhat unfair given that in this new game we, presumably, get only new characters, but I'd feel a lot safer if there was more than seven (five, not including the basics we know of the leaked two) slots to fill. I remember back in the lead-up to ME2 with the constant guessing and new character theories, the slow leak and introduction of new companions. I actually think that ME2 did a good job of balancing its larger cast, ignoring the Collector attack part. After playing 3 and the Citadel DLC, I can honestly say that Zaeed was the only companion in 2 that I hated.
So if I had to say a number, I'd say Nine. I realize that larger numbers would probably be more difficult for the writers or programmers or whoever, but I think that at least with more there is more chance of there being characters you care about or even love. I'm happy with any number between six and a hundred, really. I have quite a bit of faith in BioWare still, and I think that they can make seven work. Whether or not they do, however...