According to Apple’s App Store statistics, 85% of iOS users were running iOS 8, as of August 3. This is a huge amount, and is a key figure to understanding the Apple Music user base.
Since you need at least iOS 8.4 to use Apple Music, and it’s likely that most people running iOS 8 are running the latest version, this ensures that most iOS users can try out Apple Music. (It’s hard to find a solid number of active iOS devices, but 500 million seems to be a safe estimate; Apple announced the sale of the one billionth iOS device early this year.)
And with iOS 9 just around the corner, it’s likely that many of the devices running older versions of iOS will get upgrades, increasing the potential user base.