How many achievements exist across all Steam games?

How to get close to this number

Multiply the catalogue by the convention. Steam lists tens of thousands of games, and a game that implements achievements typically ships somewhere between a dozen and several dozen. Multiply a realistic average by a realistic catalogue size and the total lands in the millions without needing to know anything specific about Steam. This is the standard move for any "how many X across all Y" question.

The instinct that misleads is thinking about the games you own. A personal library is a few hundred titles at most, and it is overwhelmingly made up of games people actually play. The catalogue is dominated by the long tail — small releases, asset flips, abandoned projects — and in aggregate that tail contributes more achievements than the famous games do.

The detail in the explanation is the one worth keeping: some achievements can no longer be earned at all, because they required servers or seasonal events that no longer exist. A count of achievements is therefore a count of defined achievements, not obtainable ones, and the gap between those two numbers grows every year as online games shut down. It is a good reminder to read what a total is counting.