The EU has expanded multiple times since its inception.
In May 2004, ten new countries joined the European Union, bringing the number of member states to twenty-five. Since this enlargement included many Eastern European and Baltic countries, it has greatly expanded the geographic scope of the EU, as well introducing many new official languages. In 2007, Bulgaria and Romania joined the European Union. On July 1, 2013, Croatia joined the EU.
This process of countries joining the EU is often referred to as enlargement. Countries who wish to join the EU must meet the necessary membership criteria. There are 3 stages in the process of enlargement, which is subject to approval by all EU countries: (1) Becoming an official candidate for membership (2) Undergoing formal membership negotiations (3) Completion of negotiations and accompany reforms to the parties' satisfaction and joining the EU.
Further enlargement is still being planned. With the war in Ukraine, Kyiv has asked for a fast-track procedure of EU membership. The EU has also started accession negotiations with the candidate countries, Albania, North Macedonia, Montenegro, Serbia and Turkey. You can find more information about the negotiation status of candidate countries here.