This is a difficult question to answer, as there are many factors that can contribute to a company's success or failure. However, some estimates put the success rate of Y Combinator companies at around 10-15%. This means that for every 10-15 companies that are accepted into the program, only one or two will be successful.
Of course, this is just an estimate, and there are many other programs with different success rates. It's also important to remember that success is relative; a company that is considered a success by one person may be considered a failure by another.