Back in the VGA days, I worked for a video card maker, and one of them supported a multi-head configuration. You could stick as many as 16 into one machine, if it had enough slots. Thus driving 16 monitors at once. I was responsible for taking the stock Windows 3.1 display driver for the chipset and modifying it for multi-head support. That was a really fun and challenging project, and ultimately very satisfying. I am pretty sure there were a few customers who needed the full 16-card setup.
BTW this was done in the day when drawing pixels on a screen was all done via the CPU telling the card "draw /this/ pixel with /this/ color". Good times!