If you are going to launch a startup, how many friends would you need?
blogs.msdn.com
blogs.msdn.com
If you looked at a graph of the total dataset, what you'd see is that almost all startups with only one founder fail. So the reason the successful startups you list mostly have two founders may not be because two is the optimum number, but rather because two is the next smallest (and therefore most common) number after one. We can't tell without additional data.
What I find interesting, is that a comparatively large number of companies on that page had only one founder, even though the odds seem stacked against them. Maybe there are substantial advantages to being alone. For one, you avoid having to design by committee.
It is clearly a biased sample, listing only "successful" companies but not the much larger set of unsuccessful ones. The "exception" of Excite thrown in with 6 founders, IMO makes the author's intentions (conscious or unconscious) clear since one would expect some argument about whether Excite was really a success in the end.
Then there is the question of what constitutes a founder. This isn't always so easy to define. How many people founded IBM?
Despite Thomas Watson being listed as the "founder" of IBM, it is hard to even answer this question since IBM was formed out of the merger of three companies before it was even called IBM. (http://www.answers.com/topic/history-of-ibm)
Several responders pointed out correctly that the number of potential communications channels rises as the number of founders increases. And while that's clearly true, it is also true that more founders means more mouths to feed early in the company's development. And to run any business you need to learn to manage communication amongst your team.
Finally, one has to be careful not to confuse correlation with causation. Just because Microsoft was founded by two people, doesn't mean that was the primary reason for their success. It might also have had something to do with who the two people were after all. ;)
I guess the other possibility is that the sample in the article is skewed to make a point...
What's different between the very earliest stages of a startup and the stage where you've hired one person?
It's hard to say how many founders works the best. You just need to find what works for your situation. I'd say it's usually on the low side so you're not burdened with too many conflicting inputs at one time. When you have founders upwards of 5 or 6, it reminds me of high school clubs/organizations where everyone was a co-President, co-VP, or co-Secretary. Sometimes people just want to ride along and get a position, whether it's warranted or not.