It's a documented trend that more monitors enhances developer efficiency. There is a point of diminishing returns as well, so two or three is the sweet spot. Some can see improvement with four, but not many.

I apologize, I can't find the source of the study for efficiency.

That being said, some prefer a single screen. I love dual screen as I'm a big proponent of code on one screen and website on the other so I can easily see the results of my code.