U can buy another stick, but i would heavily heavily advice against it. The reason for this is normal the kits are heavily tested together. Also never mix 2 kits of memory up with eachother. Keep them always together.. If one has issue's or just a little bit of timings off, it could effect the performance again drastically or create other issue's.
I wouldn't do it. Specially not for a PC of your caliber. For somebody with a toaster that plays league or something that doesn't stress his memory it isn't much relevant. but for you it absolutely is.
Whenever u buy memory, check what motherboard u got. Go to the website of the company like gigabyte for example, google your motherboard and look for compatible memory. Pick one of the list.
Also make sure that if you have aircooler like noctua d14, that the memory isn't high but low in order to fit under the cooler if you got a smaller motherboard.
other then that u are good to go.
Also install your memory correct, so don't put them right next towards eachother. Put them like this guy does it. In the right colors, mostly 1 in between. Your motherboard manual probably has the information what is best but this is how you do it, so u can see if visually, or else your memory is still going to run in single channel solutions.
Also your CPU 8700k is still high end in this day, don't let those AMD pr machines fool you. AMD was far behind with the 2000 series on intel, the 8700 is pretty much a 3600x and the 3600x vs 5600x is what? 15% faster at best. CPU technology is laughable. if you upgrade ot even a 5950x u will see minimal gains in games even at 1080p.
Now how much memory u go for, and what your goal is for how long u going ot keep your PC before upgrading is all up to you. DDR5 is around teh corner so if you upgrade a lot, u could see that jumping to DDR5 is a better solution and not invest to much at this point in memory. However if you stick with that PC for a while like 4-5 years, i would just say get 2x16 and upgrade it later on to a total of 64gb. however we could see a huge decrease in memory recuirements if directstorage is going to work out. Who knows tho.