Resolution you game at, refresh of your monitor, type of games you play, and the quality settings you like to set in your games, are all needed to give you an accurate answer.
In short, a 3600 will be more than fine for a 2080S, unless you have some very specific scenario specified by the aforementioned requested information, then you may want a 9900k. But the chances of that being your scenario are slim to none.
CPU bottlenecks are blown way out of proportion these days.
Let us know on those questions and we'll be able to give a very accurate answer.
EDIT:
Actually I can sum this up simply ... Are you planning on using your 2080S for professionally competitive eSports gaming for large prize money? If the answer is No, then the 3600 is plenty good enough.