I have the Asus Prime Z270-P and have 6 cards running currently.  I have the ability to add 2 more cards through the M.2 slots but haven't done so yet.  The Prime Z270-A has one more slot, so can reach a total of 9 cards with the M.2 slots, so it's a little better but harder to find/more expensive.

I believe they are both relatively easy to set up and get running with multiple GPUs, not plug and play, but adjusting some bios settings and it's not too bad.