Dropbox has 2 founders [1]
I'm curious about how successful a start-up can be, before adding anyone. [problem: founders who want to grow will do so ASAP; founders who don't, won't publicise their niche and we won't know about it]
Does Minecraft count as a "start-up"? With a billion+ dollar exit, it should be a "unicorn". I think Notch was solo for a very long time.
Of course you cannot build Amazon on your own. The difference is whether you have the type of friends who can work for free for a year while they co-found a business with you (and would be interested in doing that), or whether you are capable of filling the second-head gap with a good hire.
That's a bit of a cop-out, since anyone can say it's a top priority on their app. Except not really: Drew was from MIT, and therefore had a large pool of potential cofounders to select from. Most people don't.
It also feels true to say that YC was more squeezed for talent back when Drew applied. Nowadays YC can afford to reject startups for not having two cofounders from the start, especially in borderline cases.