Upvote:3
You can apply to extend your visa while you're in a Schengen country, if your current visa is still valid.
However, you will not be successful unless there's a very serious reason, such as you becoming so ill that you cannot travel. If your visa was granted for 20 days, then the issuing country decided it should be enough, and your itinerary probably showed an intended stay of no more than 20 days. The European Commission website has a Schengen FAQ that states (Question 18):
When applying for a visa extension, you must show that due to force majeure or for humanitarian reasons or for serious personal reasons it is not possible for you to leave the territory of Member States before the expiry of your visa or the authorized period of stay