Well, I have one more monitor than Bill Gates. I have 4. Bill Gates only has 3. Nayh nayh!
4 Samsung 213T's (22" 1600x1200 LCD) on Ergotron arms. (Each arm holds 2 monitors. They make an arm that will hold 3 - but not 4 - 22" monitors. I think you can do 4 if they are smaller).
I recently took the arms off of the desktop bases and mounted them on the wall. I called Ergotron to see what I needed to get to mount on the wall. They insisted that I needed to replace everything.
So, I went to Radio Shack, and bought two wall-mounts for TV antennas. Total cost, $16.
4 is probably overkill for most people. I think 3 is the sweet spot. The problem with 4 is that there's no one screen that is your focus. It's a bit strange to sit there looking at the crack between two screens. ;) (You can also go quad, but that can hurt your neck pretty quickly...)
I did this a few years ago when I was doing software development for automated stock trading. Although I wasn't trading myself, I still had to test the application and sometimes monitor stock quotes (with a seperate app) while testing. Add your development environment - maybe two copies for cut-and-paste between projects, a web browser window, and an instances of Acrobat for reading manuals, and I did manage to fill-up 4 screens.
I'm currently reconfiguring this. I'm going to cut back to 2 screens for my Windows system and switch 2 of them to a new Linux system for web development. I'm also thinking it will be a painless way to try to transition as much of my computer use as possible from Windows to Linux.