How many countries are members of NATO?

How to get close to this number

Build it from geography. Western Europe accounts for the bulk, North America adds two, and the post-Cold War enlargements brought in most of central and eastern Europe. Counting those blocs rather than listing members gets you to around thirty, and bloc-counting is the reliable technique for any membership question.

The number moves, and recently. As the explanation says, the most recent accessions ended decades of formal non-alignment by countries that had stayed out on principle. Anyone answering from a few years of stale memory will undercount, which makes this one of the questions on this site most sensitive to when you last paid attention.

The composition detail is the historically significant one: more than half the current membership joined after the Cold War, as former Eastern Bloc states applied through the 1990s and 2000s. An alliance founded with a dozen members against a specific adversary now contains many countries that were on the other side of it, which is the whole story of European security in one count.