One. I prefer deep focus into one thing: my code. I alt-tab to get to my app. That little gesture is just enough to require me to think about what I'm doing. It's just too easy to change my glance from one screen to another; focus is ruined.

I have tried two or three screens before, but just got irritated by the noise.

I check email and hn on breaks. They don't have dedicated screens during work periods.