The larger power supplies have more pcie ports, more sata ports and more peripheral ports than the smaller supplies.
Many Risers can be powered either by 4 Pin Molex, SATA, and 6 pin Pcie. Risers also consume the least power so can be adapted with splitters if necessary.
A 1300 Watt power supply may look like this:
Which would work fine with GPU’s having one 8 pin power connector. However, to power 8 GPU’s with Dual 8 pin power connectors you’ll need a 1600 Watt power supply like this in Gold or Titanium.
GPU’s with 2 eight pin power connectors can be fed from one 8 pin cable attached at the power supply Pcie port with either the cable has 2 eight pin plugs on the GPU end, or use a bumble bee colored 8 pin to 2 6+2 pin splitter. That takes care of only one card so unless you have the 1600 Watt power supply, you will run out of Pcie ports after 4 or 6 cards depending on the power supply.
To add additional 6 pin pcie power for risers you can use one CPU power 8 pin to a 4+4 pin cable with an adaptor sold at Amazon that rewires it to be another Pcie cable. Use this with the 8 pin to two 6+2 pin splitters splitter to power two more risers or another GPU. You’ll need the other CPU port for your motherboard.
Here is the adapter:
and the splitter:
https://www.amazon.com/JacobsParts-Express-Power-Splitter-Cable/dp/B07611QXG4/ref=sr_1_16?dchild=1&keywords=pcie+8+pin+splitter+adapter&qid=1599028656&sr=8-16
Can’t guarantee the power connection is correct for the risers, but I’m pretty sure it will work for two riser cards using the splitter below with the extension cable further down. SATA or Molex direct from the power supply would also work for risers but 6 pin pcie has the best current handling capability.
https://www.amazon.com/Tan-QY-Graphics-Y-Splitter-Extension/dp/B07HKTFVXG/ref=sr_1_12_sspa?dchild=1&keywords="6+pin"+pcie+splitter&qid=1599032558&sr=8-12-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzOENGUUlNN0o4SldLJmVuY3J5cHRlZElkPUEwMzIwNjczMUhOVVFFUklMRE5CMyZlbmNyeXB0ZWRBZElkPUEwODI0Nzc1Mjk3NTlVWTlDSk5GNSZ3aWRnZXROYW1lPXNwX210ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=
https://www.amazon.com/Express-Adapter-Graphics-20-inches-TeamProfitcom/dp/B07D8QBN79/ref=sr_1_5?dchild=1&keywords="6+pin"+pcie+cable+male+male&qid=1599032796&sr=8-5
I would test it first as the positive all on one side and negative all on the other side should work for the Riser cards fed from the peripheral and sata ports on the power supply.
You could also use two smaller power supplies and the relay or 24 pin adapter to start both power supplies at the same time.
Never plug more than a 1600 watt power supply into any circuits fed from one circuit breaker.
