How many countries are members of the European Union?

How to get close to this number

Count by enlargement waves. Six founders, a handful of western European additions across the 1970s and 1980s, a large central and eastern European wave in 2004, a couple more after that, and one departure. Adding those groups lands you within one or two, and it reconstructs the history at the same time.

The departure is the detail that trips people, and it is worth noticing that it happened only once. A bloc that had only ever grown suddenly had a mechanism for leaving tested in practice, and the count went down for the first time in its history. Membership figures are usually assumed to be monotonic, and this is the clearest European counterexample.

The 2004 enlargement in the explanation is the one that changed the institution most: ten countries at once, nearly doubling the membership and shifting the bloc's centre of gravity eastward. Note also that EU membership, eurozone membership and Schengen membership are three overlapping but different lists, and conflating them is the most common error in questions about European institutions.