I currently have two monitors and find it to be more than enough. I used to trade with only one for a long time. Certain types of trading do require alot more, but I've found that information overload can be a problem for the inexperienced. My opinion is that a good trader will be good regardless of his/her hardware setup and vice versa.
Performance enhancing drugs. If your a professional football player : steroids, amphetamines " " " professional stock trader: antacids, sedatives, vodka. Me........., I'm on everything but rollerskates.
I shared this office with someone a few years ago...it was too much. Now I have 3 which is still more than enough. http://www.iwinfree.com/tradingdesk.jpg
Five, but I will ditch them for virtual reality goggles with an infinite field 3-D display using technology from Full Metal Jacket to "execute" MM's. aphie is working on it for me....