OK great - yes, all of the motherboards I have been considering support 6 or more AMD cards, so that's good news. It's hard to find a lot of info about NVIDIA mining as there's so much focus on the much cheaper AMD cards.
Looks like I might actually settle on the Asus Z270-P motherboard.
I would go with the Asus Z270-A / AR motherboard. That's the board BBT used in his 1060 video and he' built a nine card RX 580 rig with two diffrent brands of cards on that board using M.2 to PCI-E expansion cards.
https://www.youtube.com/watch?v=pQ-EAunoAqYWow that's really cool - thanks for the suggestion.
For power supplies, is it better to go with one large 1200w or two smaller 750w PSUs?
If anything, I'd go with an atx psu to power the mobo and a server psu to power your risers and gpus.