What's wrong with having one founder? To start with, it's a vote of no confidence. It probably means the founder couldn't talk any of his friends into starting the company with him.
I don't have many friends. I definitely could talk people into starting with me but I prefer maintaining control and direction of the startup.
Starting a startup is too hard for one person. Even if you could do all the work yourself, you need colleagues to brainstorm with, to talk you out of stupid decisions, and to cheer you up when things go wrong.
I hired a Personal Assistant that I get along well with pretty decently. I can build a team around me. I have just enough funds to hire a contractor for additional part time help.
I want to be a solo founder maintaining control over the startup with a team of employees around me. Is this bad?
If your goal is to get into YC, go find yourself a warm body to be co-founder or join numerous others looking to get into YC but don't have teams. Of course, as a YC alum (Nikkey or someone from Australia, can't recall exact detail) wrote in the blog post, be prepared for the situation when co-founder bales out on you after getting YC stamp.
If your goal is to have a successful startup and business, focus on growing your business, gaining traction, gaining customers, building product, getting revenue and not on getting into YC. You don't need a co-founder or YC stamp to start a business or have a successful business. While working on your startup, if you meet someone who fancies as a potential good co-founder, explore to bring them onboard, otherwise continue focusing on growing your startup.
IME, more successful people become, more tunnel vision they get. There is more than one way to any destination.
I'd also like to advise against finding a random cofounder you don't know well. YC actively tries to filter out those kinds of matchups (for good reason); I suspect it might hurt your chances more.
The reason why YC wants co-founders is that in the space of everyone that is a single founder, proportionally very little of them can actually do it alone. Compared to teams with two people.
Being able to convince someone else to work with you on a crazy idea is a proxy that there's some merit to the idea.