I can see maybe using up to 3 screens, but I have a friend who uses 6 (of various screens, i.e. an old laptop, his TV, etc.), and I can't quite see the benefit of that. For myself, I use two and I find that's quite sufficient for me
I have 1 monitor atm.
I'm going to buy a second and bigger one.
The big one I'll use for gaming and the other one (the one I use now) for live streaming some football games or skype or twitter or whatever.
I use two monitors and I've been doing that for a while now, I can't stand using only one so that's not even an option anymore. I usually have chrome up on the second one with music, a muted stream or something else.
When I edit I put the preview window there so I can edit in full screen and leave more room for the timeline on my main one. My second one is an ips monitor so it shows colors a lot better than a tn.