You must register for VAT in a country you are selling to when you reach the threshold limit.
Threshold limits are not the same within the EU but set by each country individually. Once reached, you need to register in the country of import for VAT and pay taxes there. The delivery threshold limit in Poland is PLN160,000 (ca. €40,300).
Example: If an Austrian registered online seller sells his goods to Poland, he must register for VAT in Poland as soon as he has reached the Polish delivery threshold limit.