Take these stats with a grain of salt because they’re guesses. Chandra Steele on iOS 12’s adoption rate vs. previous versions:
Over the first 10 days that iOS 12 has been available to the general public, only 20.7 percent of those with iPhones downloaded it. That’s considerably lower than the download rate for iOS 7, which saw a 63.4-percent adoption in its first days.
I suspect the iOS 7 update rate is bogus.