Does YC accept solo founders?
Yes. We regularly accept solo founders. That said, our advice remains that one-person startups are tough and you're more likely to succeed with a co-founder.
Should you have a co-founder?
Investors, generally, tend to support companies which are run by a team than those that run solo. They trust companies with multiple founders and are likely to fund them more easily. So it is best to get a cofounder or cofounders by your side if you want to make the funding process smoother.
How hard is it to get into Y Combinator?
Depending on your source, the Y Combinator acceptance rate is between 1.5% to 3%. There is no formula for getting into YC.