There are few videos on youtube floating around with people building 6-8 card rigs on Asus z270 boards. Off the top of my head, Cryptomind has one which recently posted (and has a thread about on this site).
Couple of tricks to keep in mind:
- you need enable the 4G decode option in the Mobo bios to enable more than 4 cards.
- The Asus boards will come with either 6-7 PCIE.
- These boards also have m.2 slots which can be converted to a PCIE slot with a $10 adapter.