i can suggest a mb like these Onda B250 D8P or B250 BTC Mining Motherboard
I think that Onda has 8 size x16 slots that run at speed x1. B250 chipset supports max 12 PCI Express lanes, so simultaneous operation of eight GPU's at x16 speed is not possible at all.
Oh ok, sorry i din't know , i tought they were running at x16 speed