I have enjoyed for years developing with more than one active monitor. I like having lots of windows open simultaneously. My current set-up takes this to a new extreme.
I recently purchased a Seiki Digital SE39UY04 39-inch 4K ultra HD monitor. 4K. Four times the fun of 1080p. The kicker: I got it for $450 from Amazon.com (today it is listed at $499.99).
Now, you can spend more on a 4K monitor. Lots more. And the Seiki is limited to 30 FPS so you gamers out there might not be as happy as I am. But I use it for development and I find it is crisp, reasonably color accurate (enough for me), and just the right size.
I do need to dial the brightness back a bit and set sharpness to 0. Beyond that, it worked out of the box and I am convinced it improves my productivity.
So back to our topic: How many monitors is enough? I’m not sure but this is the current setup for my main development rig:
I have a very nice laptop: Asus G750J with an Intel Quad Core i7-4700HQ at 2.4 GHz, 12 Gb memory and a NVIDIA GEFORCE GTX 770M. The laptop simultaneously drives its own 1080p display, an Acer T232HL multi-touch display at 1080p, a LG E2241 display at 1080p, a Pluggable UGA-2K-A USB graphics adapter connected to an Insignia 22" monitor at 1080p, and the Seiki 4k monitor, all at full color depth. Occasionally, there is a slight glitch coming out of sleep, but I can play videos (DVD or YouTube) on any monitor other than the Pluggable in a window or full screen easily while working. Read more about How many monitors are enough?