rx580s don't require both power slots only the eight pin one. The hx1200i will be enough to power 6 cards. I have a hx 1050 and it's doing fine with 6 rx 580s. Plug them in one at a time checking the device manager to make sure the computer sees the card then power off and add another. Rinse and repeat. Good luck. If you run into problems , check and make sure you pci-e riser is good. I have had to deal with a few bad ones on several builds. Also do not plug more than two risers per sata cable. The hx1200i should have at least 4 sata cables, use them. What kind of motherboard are you using? Asrock h81 btc pro is the best board and is pretty much plug and play.
Thank for help! I have Asus prime Z270-A and I connected 5 cards. I am not sure what to do now with the 6th one as I used one of the 8pin slots in the PSU for the processor on my mother board. Hence a question.
Can I use 1 of my 8 pin cables to connect two cards via 6+2 ending or maybe use a 6 and 8 pin cable to connect power the processor (6pin in the PSU and 8 pin in the motherboard)?