I want to know adapter is original or copy

Two things:

1. With that little amount of information, we have no idea which 20W adapter you have or where you got it from (or will get it from). If it isn't directly from Apple or an Apple Authorized Reseller, it could be a fake.
2. No one here can guarantee what you have is orignial or not. Even providing us with photos or serial numbers won't help.

Bottom line? If you are not sure, then if you haven't bought it, don't. A low selling price may or may not indicate if the adapter is original or fake either.

I agree with Rudegar - the only way to be sure is to buy from an Apple shop. Everything, including serial numbers, can be easily faked.

Why are you asking - is there a problem with the adapter.

only way to be sure is to purchase it in an apple store or official apple reseller

everything else can be faked.