In a publication aimed at app developers, Apple revealed information about the scale of acceptance based on visitors to the App Store. It says that 50 percent of all iPhones now work with iOS 13, with 41 percent working with iOS 12 and the remaining 9 percent running earlier versions of the OS. There is also information about more recent devices, like 55 percent of all iPhones introduced in the last four years using iOS 1
iPad paints a slightly different picture, mostly because of the release of iPadOS, a slightly thinner version of iOS 13. Absorption seems a little slower on this device, with 33 percent of all iPads running iPadOS and 51 percent of devices still using iOS 12.
Although it's worth taking into account the context that iPadOS was released shortly after iOS 13, on September 24th as opposed to September 19th.