It depends on the cpu and how far you plan on pushing it. For instance, with FX the Sabertooth had one plug and the CHVZ had 2 plugs. Some people running a full water loop and pushing for max OCs on the Sabertooth used so much voltage that they melted that 8 pin plug. Meanwhile I had no problems while running air cooling.
So, what are you planning on doing with your new rig?
A single 8pin should be able to provide 600+ watts. However a lot of PSUs can't provide that over a single cable. So they add more then one. However most people will not need more then a single 8pin. Unless you are going to do some ln2 runs.
Some boards used to include an extra 4 pin for PCI-E power requirements, generally for multi 3/4 way GPU's. Hasn't been needed lately due to the trend away from multi GPU. Might have to do with the additional power requirements for PCI-E 4.0...?
Asus Prime Z370, Intel 8700k @5.0Ghz, Nvidia GTX 1080 Ti, 16GB @3400 CL14, Watercooling: x2 MCP355 pumps, x1 280 on front of case, x1 360 on top of case, x1 140 on back of case, x1 240 on bottom of case
if its for the CPU ignore it, its only there to make you feel better. the mosfets will fail long before you need more than 1 8 pin. check out buildzoid he goes deep into why its stupid.
Basically all the the ryzen 3000 series models up to 12 core (most likely the 16 core part) won't pull enough amperage to saturate the single 8pin EPS on air/h2o with OC's. Even on LN2 you'd need to be pulling well over 200a which for daily use and air/water oc's is just not going to happen.
99% of boards will function with just the single 8pin EPS connected.