To answer the OP:

ASROCK AB350 Pro 4 - Craps out after 4 cards. Will POST, but Windows won't complete boot. Tried using M.2 adapters and everything, but it didn't work. Maybe in the future with bios updates.

Gigabyte AB350 Gaming 3 - 5 cards works without any trouble. I have an M.2 SSD in place, so I can't test 6, but I suspect it will work.

ASROCK x370 Fatal1ty Gaming K4 - 7 cards works, 8 won't POST (using M.2 adapters). It's a bit picky about connecting cards - Turn off the power supply when connecting a card; don't just shut the machine down. If it won't boot after adding a card, turn off the switch on the PSU and clear CMOS. That usually fixed it as I added the cards 1 by 1 until I got 7 installed.

The Gigabyte was by far the easiest to get booting and working. If you're looking for a cheap board, it's not a bad choice. The Fatal1ty is nice overall, though. CPU overclocking is far better, the two M.2 slots is nice, the extra mini M.2 for wifi is nice and the sound card is great as well.