[oneplane]

If I'm not mistaken, Apple's CoreStorage is based on SQLite, so pretty much every Apple device for the last years (or decade, probably) is constantly using tens (or more) SQLite databases, per OS. That includes Macs, Phones, Watches, AppleTV etc. That has got to be more than 3 billion devices (since there are at least 2 billion of the phones in active use).

SQLite is also quite often a CI/CD default when building and testing software where you might want to do some tests without starting an entire RDBMS server.

But I suppose it's different if we think in terms of networked databases, or multiuser.

[Twirrim]

It's also heavily used on Android, so that's some 3bn devices (based on active users numbers from earlier this year). Plus it's in Firefox, Chrome, and so on.

[oneplane]

It's probably on all Electron apps as well, and considering some of the parts of Android and Chromium are used on Smart TVs, head units in cars and game consoles, that's probably at least another 1bn.

Suffice to say SQLite is probably at least 6bn active usages in size.