which CPU to pair with this?
Although it should work from a celeron, like the PRO BTC H81 gen 4 motherboard with the G1840, for this I say better go with a good dual core processor. G4500 up to G4560 should handle all those PCIE pretty easily. In Windows though maximum Gpu-s it will allow you to use is 8 while in Linux you can do a bit more. However in Linux is a bit more difficult to configure things out.
According to Cryptomined, Windows 10 can handle 8 per driver so you can have 8 AMD and 8 Nvidia on the same rig. He also has a review of this board.