I've got six virtual desktops with two monitors. When I'm doing web development, I have one window per screen, which usually means one window of tabs connected to my dev server, and one window of tabs for each framework/language I'm working with. For example, I was working on a WordPress plugin with JQuery, so I had a dozen tabs open on the WordPress function reference, and another dozen tabs open with JQuery stuff. Then I've got a virtual desktop with my email on one screen and Hacker News, etc. on the other one.
Finally I have two blank desktops that tend to get eaten up by tabs that I use when people come into my office asking questions that I don't know the precise answer to off the top of my head.
But yes, on a single desktop about 12 tabs is my limit. And when I get over 4 full browser windows I start to get antsy and know that I can't possibly keep focused on everything I'm doing.