Y Combinator is a startup accelerator that invests in a large number of startups twice a year.Y Combinator is a startup accelerator based in Mountain View, CA. In 2005, Y Combinator developed a new model of startup funding. Twice a year they invest a small amount of money ($120K) in a large number of startups (most recently 68). The startups move to Silicon Valley for 3 months. Y Combinator is a startup accelerator that invests in a wide range of startups twice a year.