Ok. There were 435 applications for this summer. I assume that YCombinator's traffic has been growing steadily and that more hackers have been captivated by YC, so therefore more will apply. 65 more probably isn't too far off the mark.
As for the other assumption that YC will accept more than they did last time, I'm basing that off of Paul's words.
Bravo!
You should become an economist.