I strongly suspect this approach scales to tens of thousands of users. Maybe 30-40k users would be my guess on a garden variety intel i5 desktop from the past 3 years or so.

I say this because that hardware (assuming NVMe storage) will do north of 100k connect + select per second (connect is super cheap in sqlite, you're just opening a local file), assuming 2-3 selects per page serve gets me to the 30-40k number. The http server side won't be the bottleneck unless there's some seriously intensive logic being run.