AMD has just announced that they will be bringing multi-GPU support for RX Vega 64 and RX Vega 56 with their Radeon Crimson ReLive Edition 17.9.2 drivers, which were released today. The CrossFire support is for 2 GPUs only, so AMD is going the route of NVIDIA in not having official support for three or more graphics cards. AMD has provided the graph you see below showing the difference in having 1 card compared to two in their internal testing.
There are some very impressive results there showing great scaling in games such as ar Cry Primal, Metro Last Night Redux, Sniper Elite 4, and The Witcher 3: Wild Hunt. Some of which show a performance increase of more than 80%! Expect to see more test as these new drivers get int the hands of people who have two Vega GPUs.