confirmed 6 x nvidia gpu's on asrock ab350. bios 3.20, no bios setting tweaks required. install at least one card directly on 16x primary slot. do not install any cards or risers on secondary 16x slot. needed to use m2 > pcie adapter for 6th card. ubuntu 17.04.
Missed this one. Will try m2 > pcie adapter.
M.2 -> PCIe adapter didn't help. The card is not detected by windows when connected to M2_2, and goes crazy when connected to M2_1 (just the same as to PCIE4).
Tried ubuntu 17.10 and arch. Kernel panic on boot.
Also I don't recommend updating bios to P4.40. It doesn't boot even with 5 cards. Didn't try less cards.
It is stable on P3.20 with 5 cards and win10 though. So I guess I have to leave it at that.