No serious venture is plausible to tackle alone. But every venture that YC has funded could be done with a single founder. There are other people in companies

No serious venture is plausible to tackle alone. But every venture that YC has funded could be done with a single founder. There are other people in companies and they're called employees. You can get to demo day with one founder and employees. How many YC companies have failed or split or reformed because the founders visions didn't align?