They don't need to be hooked up to anything, just how many do you have in your house.

I've got 7 CRTs (1*12", 5*19", 1*20") and a 14" LCD touchscreen made for in-wall mounting.

I'm using two of the 19" CRTs on my main computer at the moment, and the touchscreen is connected to my server.

Most I've had on one system at once was four of the 19" screens, but my secondary card died since then. [proof]

I've got a quad-head pci graphics card at my brother's place so I'll probably try out a six-screen setup eventually just for kicks.

Before anyone asks, yes that sticker on the power button of the second screen says what you think it says.