Y Combinator does seed funding for startups. Seed funding is the earliest stage of venture funding. It pays your expenses while you're getting started. Some companies may need no more than seed funding. Others will go through several rounds. There is no right answer; how much funding you need depends on the kind of company you start.
At Y Combinator, our goal is to get you through the first phase. This usually means: get you to the point where you've built something impressive enough to raise money on a larger scale. Then we introduce you to later stage investors or occasionally even acquirers.
We think hackers are most productive when they can spend most of their time hacking. Our goal is to create an environment where you can focus exclusively on getting an initial version built. In any startup, the first couple months tend to be the most productive of all. We seem to have succeeded in creating a good environment, because many founders have told us that the first ten weeks of Y Combinator were the most productive period of their lives.