This can vary depending on the size/capacity of your drum sander, but generally, a drum sander will remove no more than 1/16th of a piece of wood. This means that you’ll be able to mitigate small blemishes on the wood and smooth it down, but you won’t be able to do anything about deep stains or scratches. Some sanders will allow you to mitigate these, but they’ll mostly seem fainter rather than completely erased.