Apple's redesigned Mac Pro supports using up to three 4K displays, but it isn't quite as simple as just plugging them in and seeing the high resolution glory. There are six Thunderbolt ports and an HDMI port divided between three Thunderbolt busses, and the layout isn't the most logical.
Apple says you should connect only one 4K display to each Thunderbolt bus, and when using three displays, one needs to be connected via the HDMI port. Since it's part of Thunderbolt Bus 0, you can't connect another 4K display on that same Bus.
Maybe Apple should've thought through its Thunderbolt layout a little bit more
Bus 0, by the way, includes the bottom two Thunderbolt connectors. Instead of grouping all of the ports horizontally on the Busses, Apple chose to break up the remaining ports in vertical groups; on the left is Bus 1 and on the right is Bus 2.
You can also connect up to six Thunderbolt or DisplayPort displays, four Thunderbolt displays and a single 4K display, or two 4K televisions.
For now, Apple supports only two 4K displays: the PN-K321 from Sharp and the PQ321Q from ASUS. They both default to 30 Hz refresh rates, so you'll need to go into the television settings and change them to 60 Hz.
Apple has a knowledge base article listing the finer details of connecting displays to the new Mac Pro, and it's worth bookmarking if you plan on connecting multiple monitors to your new computer.