0. Every time I get a notification, I read it. I turn off group notifications for the chats and all because of all the people with thousands of notifications because they have chat notifications on for like 20 different user groups. I never actually got into those and I'm thankful because that's insane.
But as for my email I've got 1705 unread messages.
EDIT: And the most I've ever had is around 10. I frequently check my account and even when I post something controversial (which rarely happens but when it does... ugh... flooded inbox with 7 different people's wall of text about why I'm wrong about 3D TV) it doesn't ever get higher than 10. And this is including inbox messages, badge notifications, and friend requests.