although you can, doesn't mean you will. i think you're more the exception than the rule.
even those with multiple monitors will mostly be set up in landscape, because most things are designed that way and it just makes it easier to do stuff.
the only time i've really seen monitors set up in permanent portrait mode is in hospitals when they exclusively display lists of names, and that is all. every other monitor in the hospital is in landscape.
either case, i'm not saying you can't do stuff in portrait, but i'm used to and prefer landscape as i gather much of the rest of the world is. in fact, i can recall a video posted on how we shouldn't take video with our phones in portrait lol.