ASRock To Offer Mining Motherboard for 13 GPUs
ASRock will be releasing H110 Pro BTC+, which has 13 PCIe x1 slots for GPU. This board can mix AMD and Nvidia GPUs and the board has been fitted with extra Molex power connectors to add more power to the PCie slots.
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Source: guru3d
I'm not sure how stable 13 GPUs on one rig would be.
Not to mention finding a suitable PSU.
Today if you have the cash (these things can get stupidly expensive), I don't think you won't find a PSU capable enough for 13 GPUs at once.
However I've seen some guy built a rig with 2 PSUs into the same system, a cheaper and more reasonable solution I would say. This mostly saves you the cost of an additional Motherboard + CPU + RAM and also saves you the power costs for the additional system.
Let me see if I can find one... there, find it in your own language if you please :)
PS: sorry for the late reply, I was away for quite some time :)