I have recently installed a new exhibition on a science center where there are about 90 displays, requiring different levels of user interactivity. There are 7 computers driving those displays, ranging from 8 to 18
/computer. I found that I was more limited by the input devices than by the number of displays.
Hint: check AMD W600 graphic cards. Find a suitable board, stick 3 of 4 of those in it and have fun.