Should I register in every country or in one country? How should I choose?

To comply you have to give your collection report to a country of EU. If your business is in EU, to the country you’re business is registered, and if you are from outside EU, you can select the country of EU of your choice and give them the collection reports, they will then redistribute for you according to your collection report. You do not have to register for every single country in EU you collect VAT for.