Anandtech say it has two M.2 x4 ports:
http://www.anandtech.com/show/10518/amd-announces-radeon-pro...
And on the back you can see the standard tiered screw holes for fitting different M.2 card lengths:
Ars seems to imply that there will be two M.2 ports that users can populate.
"Instead of adding more expensive graphics memory, why not let users add their own...The Radeon Pro SSG features two PCIe 3.0 M.2 slots for adding up to 1TB of NAND flash, "
http://arstechnica.co.uk/gadgets/2016/07/amd-radeon-pro-ssg-...
It would have been really cool to have been able to swap it out as needed, but I don't think that will be doable :(