At the current time, Apple has "over 4 million users" participating in its OS beta programs, according to Cook.
Public beta testers have access to iOS 12, macOS Mojave, and tvOS 12, three operating system updates that will be rolling out this fall after an extended beta testing period, while developers have access to iOS 12, macOS Mojave, tvOS 12, and watchOS 5. watchOS 5, a new software update for the Apple Watch, is limited to developers because it's not possible to downgrade the software on an Apple Watch.
Public beta testers and developers are tasked with testing Apple's software to help the company suss out bugs and improve features ahead of a public launch.
Apple did not break out how many users participate in each of its beta programs, nor what percentage of those users are developers or public beta testers, but it's probably safe to say that iOS gets the lion's share of interest.
Despite Apple's robust beta testing process, there are still major bugs that slip through on occasion, but Apple offers frequent fixes and updates for all of its operating systems.