2 sticks. I think it’s how the CPU distributes all the information over the data lines going to the ram. Four sticks can cause instability. i’m not even sure why they have four spots for ram ingaming motherboards anymore. Absolutely everyone recommends just two.
if you do a search yourself it actually gets pretty complicated. Because sometimes four sticks are better for certain situations. it not only depends on your set up, but also the games. But here is a quick article right from Corsair.
It’s also recommended that if you go from say 2 x 16 to 4 x 16 then you should buy them as a kit and shouldn’t mix a match. As I said, it gets complicated it may or may not work for your set up with the your games. But would work with the next guy’s set up with his games