Just sharing my experience on the board that I used

Asus prime z270a/AR : about 30 of them only 1 of them fail to get 7th card running, tried everything and give up, didn't try m.2 to pcie adapter.

Biostar tb250 pro(12 pcie): ordered 5, rmaed 2, in the end 5 of them works with 12 rx 580, need to shut down and wait for about 30 sec before turn it on or no display(someone told me is latest bios issues,didn't bother to flash older bios), 1 of them lag AF in windows don't know why, but running stable for 2 weeks, had to set miner priority in task manager to get my hashrate stable.

Asus rog strix b250f: 6 cards with usually 4g,pcie adjustment in the bios.

Asus rog crosshair vi hero: same as strix b250.

Asus rog maximum ix apex: same as strix b250.

Asus rog rampage vi extreme/apex: same strix b250 but with abit more efforts x299 bios are weird.

Onda 6 pcie slot motherboard( forgot the actual name): Windows slow AF, would not start up when Windows reinstallation restart, random shut down once in a week,had to use 1st gpu display port to enter bios( running 6x 1050ti,didn't update motherboard bios).

I only use those rog board to mine cos i had to rma my biostar board.

All of them are using Windows with minimum of 4gb of ram, all cards are rx 580 8gb, lowest tier processor is a g4400

Replying on phone,sorry bout the mess
Hope this helps