I've found that two has the biggest impact in regards to improved productivity. It hurts when I have to take the laptop out of the docking station and use it's "small" 17" screen all by itself. Three monitors is nice, but doesn't have the impact of two. Any more than three and I find the additional view space to be distracting. I can see how having additional screens for monitoring systems might be helpful, but I'd want them to be hooked into another machine rather than a part of my rig.
Scott E. Corbett