Currently using two (one 27" 2560x1440 as main and the old one - 20" 1680x1050 as secondary) which makes me closer to the left one than to the right. I don't see myself using more than two, although if I had the space for a third one, it would probably just sit with the desktop visible and a Rainmeter clock on it. I think about moving to a 34" 3440x1440 and relegating the 27" as a secondary. Once you get used to that amount of pixels, 1080p starts to look small; on the other hand, I do occasionally game, so 4K would be a bit of an overkill even for the GTX 1080.