Well, now it's 18,900 since you responded to me. lol
I've had over 600 before in a Yahoo! mail account, but now I usually let the number get to about 100 before I start culling the messages. Most of them are subscriptions to newsletters, replies to posts, etc.
About 20,000 over 4 email addresses
Zero. It always baffles me when I see people with over 1,000+ unread emails. How do you see the few important emails when you get dozens of spam ones every day? I just unsubscribe from anything I don't want to get.
I'm really powering through these things, as you might have guessed.
Same here, a couple hundred unread email is all I will deal with at once. Anything more and I have a tendency to use Ctrl+A then Delete to remove the lot.
Mostly, zero. Work one is clean, spam ones are untouched (who cares). My main accounts (Gmail) have a couple unread. The reason for this, is that the unread status flags it as something I haven't got around to doing. Mobile notifications can get annoying, but they get forgotten after a while and dont constantly notify, luckily.
For my personal mail box, I get about 20 +/- emails daily, and usually check emails 8-10 hours unless there is something important which need my attention.
I currently have 20,725 unread e-mails in gmail.
I don't see a problem with this. I just keep everything in gmail's inbox, and there are enough e-mails where I don't need to read anything more than the subject. Commercial mails, mails from tech sites about reviews, acknowledgement of payments, Kickstarter pledges, ... Many of them aren't worth keeping, but there's also not much to gain from deleting them, so I don't bother.
My personal email usually range between 50 to 200 unread. I get a lot of crap in there, and even with gmails decent spam filtering and all the filters I have added, I still see a lot of crap I don't care about in my Inbox.
My work email is kept pretty clean. We got a lot of orders via email, so I try to keep it as close to 0 unread as possible. I use Thunderbird and its wonderful tagging system to keep track of everything at work.