I have 12....and use them all. Generally, each system/strategy requires one machine and one monitor, I use 3 screens as my main trading, 1 for futures. Two for charting and analysis, a laptop for signals and chatting. One for news, music, and Squawk. 4 for my systems. I'd like to have more, but only as the systems I'm trading require it.
All flat screens, 4 24", 8 20", mounted on arms and quad stand. Going forward, I will probably create my own custom monitor stand that will hold all of them. Very nice setup.