Personally, I don’t think it’s the number of monitors that increases the productivity, but rather the number of available pixels.

When I first got into my office, my setup had two full-HD monitors. I hated that, because the amount of pixels they offered was actually lower than what I had on my laptop’s built-in screen.

I convinced them to replace those two monitors with a single WQHD (3440x1440) one and I love it. Now, I’m definitely never low on screen space. (It’s a Dell U3419W)