Once you start to run more than 4 monitors, most computers start to struggle. So, then you need more computers to have more monitors 🙂 And then, you need to share one keyboard and mouse across multiple computers… so you need a mouse and keyboard sharing utility, that preferably also shares your clipboard.
I just added the 5th monitor to my office setup:
All you need is to install Mouse without Borders on both machines. The utility allows you to use keyboard, mouse and clipboard from one computer on another one.
You can get Mouse without Borders from Microsoft Garage here.
