It checks the apps on the device. Considering the amount of Android devices with Google Play Services that number seems consistent.

Yeah, I can't even imagine there being 8 billion apps in total. Like, that's more than there are humans on Earth. If every 50th person would be an app developer, then every app developer would still have to publish 50 apps all by themselves. I doubt that even the studios that just rebrand and rerelease their apps over and over again get to such a quota.