Two is nice - three is even better. I currently use two 30 inch HP flat screen monitors each running at 2560 x 1600 pixels and a 22 inch CRT running at 2048 x 1536. That adds up to desktop that is 7168 pixels wide.
I sit in front of three flat screens. It is terrific. I can’t give any advice on them, though. I’m merely a computer user. My husband set them up, and now I don’t know how we ever managed with just one screen... ;-)