Well "SQLite is likely used more than all other database engines combined. Billions and billions of copies of SQLite exist in the wild."[1]

And

"SQLite has exhibited very high reliability in the field and a very low defect rate, especially considering how rapidly it is evolving. The quality of SQLite is achieved in part by careful code design and implementation. But extensive testing also plays a vital role in maintaining and improving the quality of SQLite."[2]

[1] https://www.sqlite.org/mostdeployed.html

[2] https://www.sqlite.org/testing.html#summary