Most desktops can support two heads. If you are paying someone to be on a computer most of their day, for the extra cost, they should have at least two screens, if you can, three. I personally do start to see diminishing returns past three. Or you can go overboard and use threeXthree to make a 9 tile display.
My $0.02 for the day. I can't recommend the extra real estate enough. Once windows start overlapping I slow way down and lose focus.
|Brock's Work Desk, Laptop+TH2Go 3x19" 4x3 Screens|
|U of M 4K display 3 GPU Desktop w/3xTH2Go|