Wow, there's so few solo founders, and so few non-US companies. I bet nowadays, there's very few non-AI companies. So if you aren't a US-based AI-startup with multiple founders, perhaps ycombinator is not for you.
YC has a known bias towards multiple founders. They accept solo founders, but solos have to stand out more or maybe already started their business. YC even provides a co-founder matching system (of unknown efficacy).
PG said "Empirically it seems to be hard to start a startup with just one founder. Most of the big successes have two or three. And the relationship between the founders has to be strong. They must genuinely like one another, and work well together. Startups do to the relationship between the founders what a dog does to a sock: if it can be pulled apart, it will be."
These are two well-known biases of YC. I believe the reasons are the following:
* YC founders are usually very young, and young people can be capricious. They don't want a solo founder just deciding to do something without a second founder having some leverage to protect YC.
* YC is in the US and has a very pro-US perspective, so they invest in what they know and like.
If you have a cofounder it forces you to get in the habit of delegating really early on. Prolific solo founders like pieter levels just do everything themselves, which is great for their ego but not enough to build a venture scale business.
They have a disproven ability to find people and rally them around a common cause though. Whether it's due to their personality or the idea or some other thing like timing, the inability to find a co-founder could be considered signal, for some.
