One 43" 4K monitor (or TV) has been my go-to for the last 4 years. Plenty of screen real estate without the physical limitations of dual monitors in different orientations. And at 43/42 inches, PPI is just right for me - no scaling necessary.
Most modern units have game mode and/or PC mode which I assume disables some processing to drive down lag. On my LG C2 OLED, changing the input type to "PC" results in a visible flicker (just one flicker) when the display is shifting into some new mode.