I think Satoshi Nakamoto was a single brilliant programmer. For one thing I would expect that if there were two or more people one of them probably would have talked by now. As the old saying goes, "Three may keep a secret, if two of them are dead.". Besides that assumption there are other pieces of evidence to support Satoshi Nakamoto only being one person. For example, Stefan Thomas graphed the time stamps of more than 500 of Satoshi Nakamoto's forum posts and hardly any posts were made between 5-11 a.m. GMT. That fits a pattern of only one person posting. A second or third person would have filled in a little bit of that gap by posting on the forum while the other one slept.
