Current tab count...17 tabs in 3 firefox windows...and I frequently double that.
But it's all very organized. I like to work one job per window, using as many tabs as needed in that window because it does not close until the job is finished (or I'm happy with the progress made in it). For example, if I was writing a new blog post, I would have everything I need to research and write it open in that one window - keyword tools if needed, older posts, forums, other sites, my PRL articles source, my source of photo's, affil link details, my backlink builder and so on.
It can get a little carried away at times - like when I'm looking for something new tp promote & have 4 different sales pages open trying to find the "perfect" product and/or I'm inside the membership site I use for PLR articles and have 5+ different articles open trying to find one that isn't fluffed up bullsh*! - but overall, one window per job works for me.
I find it motivates me to hurry the hell up because until I finish what I'm working on, that window stays where I can see it. Then I can go and check out what's happening in other windows. I have one dedicated to email accounts, one for playing in (Facebook, my personal email account, games, the kids stuff...) and usually 3ish jobs waiting in their own windows for me at any given time.