After spending far too much of my life on this I found this thread.  So I decided to post my story.

I have an asrock ab350 gaming k4. I have a mixed rig. I had 4 1060s and 2 580s to put me under my 1000w psu.

I originally bought two of these rigs.  My first had no problem with 5 gpu and is still running 5.

The second rig I fought with.  And fought.  And fought.  It was 3 months ago so I don't recall the details but I believe  I was able to get 6 cards running by installing drivers one by one and mucking with slots. After it was running it was totally stable and never did the black screen booting windows ->recovery. .  

Last week a fan died in the second rig and I swapped a 1060 with another 1060.  Nothing else changed and now the machine will not get to windows with more than 4 cards. I had forgotten about the initial pain of setup and now I'm pretty pissed.  

However, I did have  a m2 to pcie card laying around. So props to the previous post about trying that. I originally stuck it in m2_2 -and windows didn't see the card.  So I tried m2_1 and moved one gpu out of pcie 4. Now I have 5 cards running.

Later, I might try to get the 6th working.  I'm thinking if I boot to safe mode and install the drivers for 6th I might be able to get it going.