ASUS PRIME H270-PLUS 109$
MSI H270 TOMAHAWK ARCTIC 125$
ASROCK H270 PRO4 112$
both 1151 socket & 6 PCIe 3 slots (same Offer as Biostar which should be much cheaper), reference price from AMZ
Gonna be hard to drive 6 GPUs from 5 PCIe slots.
Please only recommend boards you know work.
For instance I have a board with 7 PCIe slots and 2 m.2 slots that theoretically could drive 9 GPUs.
Realistically it can only drive 4.