Ars Technica/Andrew Cunningham:
"AMD’s GPUs support up to six displays, so you can use two of those connections for one 5K monitor, two of them for the other 5K monitor, one for the laptop’s internal display, and still have one left over for yet another monitor if you really wanted to use one."
Bodes well for the new iMac 27 with AMD GPU and 4 USB-C ports; 5K internal plus 2 x 5K external. All iMac 27 (5K) use a custom Apple chip for 2 x DP 1.2. Currently, only the Mac Pro (Late 2013) can drive 3 x 5K displays (2 GPUs, 6 x Thunderbolt2).