YouTube tech reviewer Adam Matthews has discovered that the Mac Pro is capable of driving six 4K monitors using just the built-in graphics cards.
Since Apple lists the Mac Pro as only supporting three 4K monitors, Matthews went about developing a custom external solution to support the additional three monitors. He purchased a Corsair CX750 power supply, an AMD7970 GPU, and a Sonnet case to make it happen.
Supposedly the Mac Pro 2013 trash can can only support 3 x 4k monitors so i built an external GPU (eGPU) via thunderbolt. It uses the Sonnet Thunderbolt chassis and in this case an AMD 7970 GPU. Although you can use GTX970, GTX 980, Titan etc. This is how i did it.
To Matthews’ surprise when he tried to plug his six Dell P715Q displays into the Mac Pro directly, it actually ran all six at 60Hz.