A good definition of what YCombinator is:
A three-month long program to help start startups. Founders get ~$15-20k in "seed" cash so they can live without any other commitments for the three months, in return for ~6% equity in the business.
During those three months there are weekly dinners with the ~15-30 other companies accepted into the programme. Investors (sometimes) attend these dinners as well. YC specifically doesn’t offer office space, but these regular dinners and office hours with Paul Graham provide regular contact and guidance from other startups going through the same issues you are. These dinners also feature guests/speakers from across the startup/tech industry.
At the end of the three months is a Demo Day, which is attended by some of the top VC’s and angel investors in the US. So in addition to helping get your startup and demo ready, YC puts you in touch with an incredibly ecosystem of VC’s and advisors to take you to the next stage.
"How much funding " you can get depends on how well your pitch at demo day is accepted by attending VC's. YC provides "pre seed" capital, product insight, company structure guidelines and network connections - you have to get the next round.
Here's a Detailed evaluation of YC from a seed accelerator program vantage point.
When you have a hot startup... other angel investors will approach you, as well as some VCs.
I am not sure if YCombinator will invest (again) in your company but what I know is that if you are one of the startups accepted to YCombinator, the exposure, the skills and the connections you'll get are usually sufficient to take your company to the next level... for many people that means getting additional investment, more likely an angel round, and for others it means to start getting money from clients and work towards profitability.