There is probably an upper bound on the number of monitors that one can comfortably and actively use, though I do not know how high it is. I've noticed that with more monitors that one is more likely to place static informational windows (social media, system monitoring, email/instant messaging) that aren't really necessary as an immediate concern. I'm not sure if it can be exploited to put valuable information there, or if one can train themselves to actively use it for work.

Managing windows and workflows with more monitors is also quite a bit more difficult than it is with a single screen. I've had some success at reducing wasted space by using a tiling WM, but having to learn key combinations to navigate is tedious.