according to apple specs http://support.apple.com/kb/HT5918
"You can connect up to:
Six Apple Thunderbolt Displays (27-inch), Apple LED Cinema Displays (27-inch), or third-party Mini DisplayPort displays.
Three 4K displays: two connected via Mini DisplayPort and one connected via HDMI.
One 4K Ultra HD TV or 4K display using HDMI and four Apple Thunderbolt Displays (27-inch), Apple LED Cinema Displays (27-inch), or third-party Mini DisplayPort displays.
Two HDMI (HD or 4K) devices: one connected via HDMI and one connected via Mini DisplayPort with an HDMI adapter (see note below)."
and they also not recommend to connect more then two displayport adapter to the mac pro...
and not more then 2 display to each thunderbolt bus
better take a closer look first there...
but maybe you need to buy everything and give it try first
there is also an spec overview for other macshttp://support.apple.com/kb/HT5219
at point 18. you find the supported display spec.
and i dont if there is an other possibility in counting Six Apple Thunderbolt Displays as 6x, 12x normal displays