As Apple describes on its recently updated Apple Developer Support page for the App Store, iOS 12's adoption rate has been quite successful since its launch last month. As of October 10th, 50% of all iPhone, iPad, and iPod touch models compatible with iOS 12 that have connected to the Apple App Store have downloaded and installed it.
This rapid kind of update adoption in the span of a month is quite impressive, especially when framed against its main competitor in the smart device OS space. According to Sarah Perez at TechCrunch, the latest versions of Android, 8.0 and 8.1 Oreo, have been installed on only 19.2% of compatible Android devices. Of course, this is likely due to Google only directly pushing the update to its own Android One, Nexus, and Pixel devices. Devices manufactured by third parties have their updates primarily managed by the manufacturers and cell phone carriers.