First China Now South Korea banned bitcoin now many people think bitcoin price may fall down and they sell their bitcoin
As far as I know, South Korea hasn't ban bitcoin yet. They maybe heavily regulated it but it doesn't mean that they are putting a complete hammer ban. Just be careful on what you post without any source. This is consider a fake news in my opinion.
Should I keep my bitcoin, sell it, or buy more?
Just hold on it and purchase if you have extra funds in you. Sooner or later bitcoin will go on another bull run and can even touch new all-time-high. No need to dump it just because of some negative news. It doesn't make any sense at all. And besides, you are not losing any profit if you are just holding, right? So why sell it.