I haven't seen statistics on single founder vs multiple founder startups. I'm not sure if there are any, or the massive biases they would exhibit because most startups die without ever leaving a trace.

Most of the companies you've worked for had multiple founders - to be get big enough to hire someone is a measure of success. I think history likes to write stories about single men of vision - especially in the established business fields. I'd take such a look with a healthy skepticism.