A lot depends on the manufacturer of monitors. Clearly, Intel is backing this as the display technology going forward and Apple is also clearly on board.
I think it's early to talk about what a 5k display means - so forgive me if you have a specific resolution and monitor in mind. Apple pulled punches to make their 5K iMac work since the GPU/controller/cables are all internal and they don't have to interoperate. The industry will have to standardize a bit for us to all be comfortable saying 5K display in general.
Currently, you need two cables for the Dell 5K display:
Some displays with resolutions higher than 4K require two DisplayPort cables to connect the display at full resolution. With OS X Yosemite v10.10.3 or later, the Dell UP2715K 27-inch 5K display is supported on the following Mac computers:
- Mac Pro (Late 2013)
- iMac (Retina 5K, 27-inch, Late 2014 and later)
- MacBook Pro (Retina, 15-inch, Mid 2015) with AMD Radeon R9 M370X
That being said, the Late 2013 Mac Pro can drive three 4K displays and doesn't have any of the Thunderbolt 3 advances, so it's inconceivable to me that you won't get two displays over TB3 at 5k and 60 Hz which is currently the gold standard for video workflows that demand high-k displays.
Since nothing is released from Apple saying they Support 5K, you're going to have to read the tea leaves and wait for shipping displays and shipping chipsets with Thunderbolt 3 to know for sure, but I see no reason to be pessimistic about what you want happening on any Mac that ships this fall assuming Thunderbolt 3 over physical USB-C port is part of that Mac.
Intel states that you get 2 4K displays per TB3 connection - so I would presume one TB3 per 5K display would suffice. The single TB3 is listed as DisplayPort 1.2 but has 8 lanes of DisplayPort 1.2 (HBR2 and MST) so I think it's really in the monitor maker's court to get as much of their 5K from one Thunderbolt 3 channel.