Never touch any adapter if you don't want to burn your house down!

1 slot in your PSU for 1 slot in your GPU, if you want to make it correct. So you can fire up 4 GPU's. A 5th card if you use the last 9th PCIe and a good (not cheap!) CPU/PCIe cable.

Many ppl will say "Hey, just use 1 cable for 1 GPU. Get any cheap chinese adapter and its fine."... Never do this! These cables were made cheaply and get very hot and if you are unlucky, it can burn.

Edit: Yes, just connect 1 of these double cables (6+2 and 6) into 1 slot in your GPU, forget the 6 Pin.

DON'T split the remaining 6 Pin with an adapter into 8 Pin and connect the second slot of that same GPU!