Right now I've got a single 28" 4K and since my eyes aren't that good anymore (approaching the slippery side of 60 here...) I had to crank the screen magnification to 150% and increase the default font size from 8 to 12 points, thus losing most of the extra real estate. Sigh...
That's me. 2 24" landscape, one 28" portrait and I'm back where I was with 2 21"s and better eyes.