In this article, Graham says "about 85 percent of the companies eventually receive further funding". Elsewhere, on this message board, he has put the number closer to 1 in 8. In 5 minutes of Google searching, the best I came up with was:
You're probably right, but an angel offers a 6-12 month runway, and YC has a longer track record than that. Graham gave a more definitive answer on the number of YC investments that had taken A rounds, but I haven't been able to find it.
http://news.ycombinator.com/item?id=145760
Which says (non-authoritatively, and in paraphrase) "50% of the minority of companies that we fund that want VC score it".
I'm sure there's a semantic thing going on here that reconciles these figures, but I'm wondering what it is.