Apple shared the latest data in a post that was published on its developer page. As of January 1, 2019 75% of all iOS devices were known to be running on iOS 12, with about 78% of iOS devices that were 4 years old and latest ones were on the newest release.
About a month ago, Apple shared that 70% of all the iOS devices were running on iOS 12, with 72% that were sold in the last 4 years being on the latest version.
The 70% for all devices saw a 10% rise from October, so adoption has actually been steady at about 5% for the last few months.
For comparison, iOS 12 adoption has clearly been able to outpace iOS 11 which was at 65% in January 2018, and is right about in line with iOS 10 which was known to be at 76% on January 5th, 2017.