My work setup has two monitors in a horizontal layout.
Left (in front of me) contains the main stuff for my task at the moment. That's where my meeting app goes as well so I can look straight at the webcam during meetings.
Right has the supporting stuff, reference docs, IM just in case I need to be pulled away for some critical issue, etc.
At most, I can work with three monitors for increasing productivity in terms of screen real estate. More than that would be a case of diminishing returns for more physical space taken.