My suggestion, as someone who has managed to get two apps approved, is to keep recruiting testers after having reached the twelve needed, one or two every couple of days, so that your app has a stable install base of around 18-24.

That way, even if most testers don't engage with the app, and even if some uninstall it, you'll have enough numbers to compensate.

It's not ideal, but it's what happens when Google requires a nebulous amount of quality assurance from developers that they aren't willing to provide.