I just joined the inbox zero club last week. Feels so good. I still have about 10000 unread mails in my personal account though. Most of them are from the mailing lists that I forgot to unsubscribe from.
1 unread email right now, but 150 new conversations in the past 24 hours. I basically spend my days on email. I couldn't imagine how I'd cope without inbox zero.
The query "is:unread -in:inbox" will find all the unread email that's filtered out of your inbox. It might not answer this question though -- Gmail may still report the total as "hundreds" or "thousands".