So I finally gave in and went out and bought myself a second monitor. I don't know why I waited this long! This is amazing. No more alt-tabbing to look up documentation is great. I can now actually have a project open full-screen, and be reading docs at the same time! So cool. Not to mention I can now watch netflix while I play D3 :D
If you give a programmer N amount of monitors, they will find at least N amount of programs to have open across all monitors at once. It doesn't matter how much real-estate you have, you can always justify having more ;)
I've never needed more than one and never needed anything over 19". So I have a 19" AOC monitor. Though, I could use a new tower as the fans run hard when I do almost anything (especially Minecraft or such...but it is only 6 years old :-/).
I had a chap on my team at work a few years ago who only used one. Watching him context switch between windows too big to have both on the monitor at once was painful. I got him another monitor and the improvement in his "flow", so to speak, was very clear to see.
Nobody needs more than one; we could do it all on a single 14" VGA monitor. It's just far, far easier to do it on (at least) two big monitors.
I literally won't take a job somewhere that won't supply at least two monitors (which, to be fair, has not been an issue for almost a decade now). Not only is it ridiculously frustrating to have to work on such a small amount of screen space, but the corporate short-sightedness of it is a very bad indicator of company values.