Despite my best efforts, with my Asus Zenith Extreme x399 down (So sad about this) along with the 60 lanes from the 1900x, I had to resort to using the only other thing I had on hand, A Z390 w/ new 9900k. Now I tried every conceivable combination of 1x risers, M.2. to 4x risers, I could not for the life of me get the second rig to fold reliably with more than 3 cards, it just doesn't happen. I managed to get GPU-z to see 5 cards using the 3x16x slots and both M.2. to 4x adapters, but it was very unstable. Thing is, with 16 PCI-e lanes for the CPU, you are left with limited other options. I know that there are 8 additional lanes on the SB, 4x of them being used by the 3rd pci-e 16x slot, but again, despite my best efforts I could not get them stable. So, until I can get a replacement x399 board, the 2nd folding rig will only be using 3 Tesla's and the iGPU to drive video.
I even tried adding a 6th Tesla to the working X399, but that resulted in the board actually not seeing 3 of the cards. With the GT710 and 5 Teslas, I think it's safe to say I reached the capacity of the board. Now, the board I have only has 1 traditional M.2 slot, which I am using, there is 1x Vertical M.2. but it is a different plug style compared to the risers I have.
So I will be looking for an X399 board that has to have 5 PCI-e 16x slots. A care package from PPCs came today.
And the above is why I NEED a 5 16x slot MOBO. There is a server board on ebay, I am probably going to get. I will be modifying one of my mining chassis to run the other 5 teslas under water. Really looking forward to that build!