Single. 19"? 21"? Not sure, doesn't matter.
Since I work primarily on flow diagrams and zoom/pan works like a dream with my wheel mouse, I have no problem with that.
Futzing with hardware/software to make everything work right is a PITA, so I like to keep it all as simple as possible.
Then again, I'm a CAD dinosaur (from the DOS days) and can make do with whatever is at hand...