use 1 cable per riser. does not matter what type of cable you use to power the riser (molex or sata or 6pin) as long as it has 2 yellow wires. but use 1 PSU connection per riser or use the VGA cable to power riser & GPU. if you need to split the 8pin GPU, buy splitters from ebay, https://www.ebay.com/itm/5-pack-PCI-E-8-pin-to-2x-6-2-pin-Power-Splitter-Cable-PCIE-PCI-Express-5X/142553881213?epid=21005666775&hash=item2130df9a7d:g:X8YAAOSwaj9dAJKX

power supplies are most efficient at 50% DC power draw from your components. so you should make sure each set of GPUs is using 50-60% of your PSU max. thats the safest and most efficient use of the PSU