You don't want to run PSU's close to the rated the capacity. They won't be as efficient and it will shorten it's lifespan. 50% to 80% of the rated capacity is where PSU will run most efficiently and cool. The other consideration is how many PCI-E connecors you need for all the GPU's and risers. Many PSU's less than 1000W will only have 4 PCI-E connectors per PSU. For 13  RX 570's you will need a minmum of 13 individual 8-pin connectors just for the VGA power. Some cards like the Nitro+ also have a 6-pin connector that may be optional. For risers I use 6-pin PCI-E risers only and connect two or three risers per PSU 8-pin cable using a PCI-E 8-pin to dual 6+2 splitter cable, figure another 5 PCI-E PSU ports for the risers, so in total you need a minimum of 18 PCI-E ports.