The success of your closed testing hinges on the quality of your testers. Simply getting 12 random people might fulfill a compliance checkbox, but it won't give you the actionable insights needed to truly improve your app.
Start by defining who your ideal users are. What are their demographics? What problems does your app solve for them? Tailor your tester search to match these characteristics. For example, if your app targets gamers, seek out active mobile gamers.
Android's fragmentation is a double-edged sword. While it offers wide reach, it means your app needs to perform across various devices, screen sizes, and OS versions. Your testers should ideally represent a spectrum of these conditions. Don't rely solely on flagship phones; include older models and different manufacturers.
Passive testers provide limited value. Seek individuals who are genuinely interested in providing feedback. Look for those who are articulate, observant, and willing to spend time using your app critically. Community forums, social media groups related to app development, or even existing user bases (if you have other apps) can be good places to start.
Remember, the goal is not just quantity, but quality of feedback. A smaller group of highly engaged, relevant testers is far more valuable than a large, unmotivated one.