Running a single 27” 4K (at 200% DPI) and couldn’t be happier.
I have a 38inch Alienware AW3821DW and two 4k dell monitors. I have the ultrawide in middle, other two on both sides oriented vertically, this config gives a lot of work space with moderate head movement.
I like to keep the laptop screen because it is a much better screen than almost any affordable monitors out there. The laptop screen is also a convenient sharable screen for video conferencing. The external monitor is just there to give me a bit more screen real estate.
I have main computer direct to lefty and a switch to go between multiple PCs (including main) on righty, do a lot of tinkering, programming, and stuff so the main might be videos or docs while the secondary is being worked on or visa versa.