I've tried two separate Z170 boards now and the result for each is the same: if I connect all (6) Sapphire RX480 8GB cards and boot, I get a linux kernel or driver crash.
First I got this board and could only enumerate 3 of 6 cards:
Next I tried this board which was slightly less terrible but still only gave me 4 of 6 cards:
Some people might ask why I opted for more expensive boards as opposed to some cheaper builds that are possible... my thought was that mining software is likely to evolve over the next few years so I wanted to future-proof it by getting some fairly high-end parts (i7 6700k, 64GB DDR 2400). Yes, I realize that's overkill but if someone comes out with a more memory-intensive or CPU miner that will run in parallel with GPU mining, I'd already be set up for that. Anyway it's neither here nor there, except now I have a socket 1151 CPU and high-end RAM sticks that I need to put into a board which doesn't suck complete ass.