IMHO too many people are talking about it Someone needs to make a video.
It really depends on the CPU. Right now, the ONLY RYZEN CPU that needs more than a single 8-pin is the Threadripper. That is, unless you're SERIOUSLY overclocking that Ryzen 5 3600 (like over 4300 MHz).
If the PC works with just the 8-pin, then that's it. You're good.
If the PC does not work with one, use two. But this is very, very rare.
I know some idiot SI's in China that will take the existing single EPS12V and split it into two using a splitter, This is dumb because you're not actually able to draw any more power from the PSU. You're drawing the same amount of the PSU and putting the same amount of stress on the PSU. You're just adding a couple additional potential points of failure. And if the board DOES need two have both connectors populated and you use that adapter, then you're really in for trouble because now you're exceeding the spec for the power supply!
I have used pigtail EPS12V cables before (where there's a second EPS12V on the cable about 10cm from the first), but these ALWAYS have 16g wire. You'll find 99% of the time that your single EPS12V cable is only 18g wire. Even on the best PSUs.