If you held a gun to my head and forced me to make a guess I'd say you could push that approach to order of 100K, maybe 1M documents.
If sqlite had a generic "strictly ascending sequence of integers" type[1] and would optimize around that, you could probably push it farther in terms of implementing efficient inverted indexes.
From my experience, SQLite's FTS5 is orders of magnitude more performant than that, i.e. for 100K documents, 7 queries/second on some of the cheapest 1 vCPU Virtual Machines.
But it is true that a specialized search engine using a more clever algorithm might be another order of magnitude faster.
If sqlite had a generic "strictly ascending sequence of integers" type[1] and would optimize around that, you could probably push it farther in terms of implementing efficient inverted indexes.
[1] primary key tables aren't really useful here.