No, I don't mean winning by one vote, Bel. What I mean is, against all odds and numbers, the majority is formed by a single person. Take a king, for instance, with complete power. That would be a majority of one. Or let's say in a single-parent family in which that parent refuses to listen to the wishes of the children ever. That would be another majority of one. At least, that's how I'm taking the expression's meaning. I could be wrong here, but that's how I'm taking it.

Thanks,
WW