Do I have to sell in all countries or is it possible to sell in only one country?

It is possible to sell in only one country. However, you won’t be able to benefit from PAN EU. If you want to use pan-European shipping (PAN EU), you need to sell in all countries where you previously had an active listing (see question about active listings).
Our Sales or Customer Success team will be happy to help you determine in which countries to sell.