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?