The cables that came with the psu, should be marked/stamped PCIe
My pref. is to use 2 cables.
In case your PSU doesn't have two 8 pin PSU to GPU Power cables (PCIe) or the GPU card didn't come with it own then you can purchase them at Amazon and other retailers.
You can purchase a 6 Pin to 8 Pin PCIe adapter cable or various PSU type connections to PCIe adapter like Molex to 8 pin adapter cable.
Note: From previous AMD Thread, Users have indicated it is best to use two different PCIe cables to connect your GPU card.
Here are a couple from Amazon.com: https://www.amazon.com/StarTech-com-Express-Power-Adapter-Cable/dp/B001TK3TJY