Mac Pro Late 2013 doesn't support 6 Apple Cinema Display?

I have Mac Pro Late 2013, and 6 Apple Cinema Display(23" x 2, 21" x 4), I connected all of them to Mac Pro through DVI to Thunderbolt 2 adapters, but It seems like only 2 monitors are supported.

I checked the specs of my Mac Pro 2013, it should support 6 monitors. so just want to know, why and how can I connect 6 monitors to it.