in theory you can run as many gpus as chipset allows you due to the number of pcie lanes.

try lowering clocks a bit step by step and see if it's getting stable.

rx 480 8gb , ~720-730 per gpu, about 80 Watt due to afterburner info.

i had some experience with those switches and some old mobos for a couple of last weeks. neither g31 and g41, nor h61 and some too old chipset mobos didn't see any cards through it, but h81 works well. i don't have any amd mobo, but due to their specs it should work too. well, getting a cheap 2-pcie slot h81 mobo to build a 6 gpu rig looks like a good plan now.

so, the thread question.
i have lots of old g31, g41 and h61 mobos around with only one pci-e x16 onboard, and i thought buying new one for rig is just waste of money. i've unsuccesfully tried couple of g31 mobos with win7 - no gpus seen.
did anyone make it work with some of those chipsets?

looks like you overclocked it too much, try lowering down freqs until you'll got stable.

i have h81 pro btc with 6x480, all rxs are connected via x1 usb powered risers. rig works fine, the only issue is that mobo wont start until i plug monitor into the x16 slot card (still x1 connected actually). i dont know, what the heck is that but monitor is always here, so i dont really mind about that.

is it a monitor connected card? if it is, try to reconnect it couple of times. i know that sounds weird but i had this issue with my nitro+ 480 8g, and it helped. think of some driver bug.