It looks like one W6800X Duo should be able to connect 9 displays but the white paper says only 8?
It looks like two W6800X Duos should be able to connect 14 displays.
This document https://support.apple.com/en-ca/101868 implies there's a MUX that switches one of the ports of Thunderbolt Bus 0 with the HDMI port. This is not shown in the white paper except for other MPX modules like the W5700X, W6800X, and W6900X. It would explain the 8 display limit though. Then two W6800X Duos would be limited to 12 displays.
Theoretically there's 6x4k on a single w6800x, so while you'd expect a single w6800x-duo could do 12x4k on its own, it would be unsurprising if it was set up like the 2013, where only one of the two GPUs actually has display duty.