I have one screen. My desk has little room, so 2 is out of the option. Also, if I had the money for another monitor, I would most likely spend it on internal components.
Honestly, if I had the money, I would have 6 screens.... 3 side by side, then a row above, I am always doing something and could use the extra screens lol.
I used to have a dual screen setup with 2 17 inch LCD's, but that was 8 years ago, since moving to the larger iMacs (had a 24 inch, now a 27) I find that its big enough for my home use. For work, thats a different story. Running a multitude of apps, all at once was a headache, even with 2 screens, but with some nice updates to OS X, its made my life much more productive with only one screen:
I can customize that anyway I want and have as many as I want. With simple gestures I am in and out of whatever space I need to be in. It doesnt help when you want to have things side by side, but I don't find I need that all that often. I think it all really depends on what you do with your computer to say how many screens you really need. But more is never a bad thing