I use one - my laptop screen

...or occasionally 2 (projector/TV etc for movies, presentations, etc)

I value mobility more than multiple screens ... especially since I can just swap either between virtual desktops (yes - that includes remote access and VDI tools like VMWare Horizon and terminal windows, but also local virtual screens) or just multiple windows/tabs in a given application

I think it's more a user problem, personally

I can manage to be as productive on a single physical monitor but with relatively-judicious separation of tasks via applications and windows/tabs in them as my coworkers who have 2, 3, or even 6 screen setups

Software can help .. but if you have an inefficient personal workflow / mental process, all the software in the world won't help :)

- I never have to go hunting for my cursor (it's always on the same screen)

- all necessary (mute everything unnecessary) notifications and associated responses happen in one place (not notifications here and responses there)

- I split activity via "smarter" use of application windows and/or tabs (keep all of X type thing in one browser, for example, with Y type stuff in another browser)

- I use colors in Terminal to show what windows (with associated tabs) are doing what

All of my coworkers split these different categories of tasks onto separate monitors - making context switching much slower (first command-/alt-tab to the right application, then figure out which screen it's on, then figure out which window/tab in that application, then go refind whatever app they were in previously to get data needed for a new request/task/etc (which repeats the find-the-app-find-the-screen-find-the-cursor-find-the-tab/window process))

My process has worked quite well on both Windows and Mac machines - or Windows-on-macOS-via-VDI (which allows another level of task subdivision/separation)