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.
actually, most of the companies I've worked for were started by an individual, and most of the successful ones were as well.
But my point is that there has to be someone in charge. You can have "cofounders" so long as they know that the buck stops with you. (Or you can be a cofounder so long as you know the buck stops with someone.)