Device42 tracks which devices are running which operating systems. This module also ties into Software License Management so that you can see how many copies you are licensed for and how many you are actually using.
The Operating System list page shows all of your operating systems, the total number of devices running the o/s (“Total count”), the number that are licensed (“Licensed count”), and the number that are not licensed (“Not licensed count”).
If you click on one of the listed operating systems, you will see details including which machines are running the o/s.