Maybe even more than 500. [link eludes me] But somewhere it was written that the Cambridge session was the unpopular session. And since May 2007 YC has been featured in Newsweek, in addition to getting a groundswell press from each individual company (eg. Weebly in Time).
Certainly not hard evidence but the limiting factor seems to be good applications, not how many they're able to accept. That's a great sign.