During its Q2, 2018 earnings call, Apple’s CEO Tim Cook revealed that the company has over 4 million public beta testers for iOS, macOS, watchOS, and tvOS combined.
Apple debuted its public beta program a few years ago as a way to ensure its software is tested by a large number of people who are willing to run a beta build of an OS on their device and report bugs back to them. By doing this, the company is able to garner a wider feedback about the quality and stability of its software, with public beta testers also coming across a far greater number of bugs than Apple’s internal QA team.
The company had started iOS public beta testing due to Apple Maps.
Apple did not provide individual numbers for each of its beta programs but it is a given that its macOS and iOS public beta program has the largest number of beta testers.
While a public beta testing program might have led to more bugs being reported to Apple, the quality and stability of its software has not improved dramatically as many would have hoped. Apple is currently running the iOS 12, macOS Mojave, watchOS 5, and tvOS 12 public beta program thereby giving advanced users an option to try out the upcoming releases of its software ahead of time. You can find the steps to join the iOS 12 public beta program for iPhone or iPad here.