The most extended model for this kind of apps is "per user".
What does it mean? Very simple: if you have 2 Android devices with the same Google account that you used to buy the app you don't need to buy a second license. If the 2 devices use different Google accounts then yes, you need 2 different licenses.
Some users said that simply adding as a second Google account the one used to by the app on a second device, the app can work with a single license. A small group couldn't... and finally a small part of them needed to reset the terminal after this test (depending on device model).
This license model is also used by Apple at their AppStore.