How are /dev/block/dm partitions determined on Android?

I was looking through my Android device's partitions and noticed that the root directory / is mapped to /dev/block/dm-5. Cold starting and rebooting the device did not influence this partition number. However, updating the device or in general, flashing another image did cause it to change. Using the same image on two different devices did not give the same results, leading me to the question: How are these partition numbers determined, and what are the permissible values (e.g. / is typically mapped to .../dm-5 to .../dm-9)?