Android - How many devices does one purchase of Android app support?

There is no limit. From Google Support:

Downloading previously purchased & downloaded apps

When you download a paid or free app on Google Play, the app is connected to your Google Account, meaning that you get several benefits:

• If you get a new device, remove an app to free up space, or reset your device, you can reinstall the app without purchasing it again.
• If you have multiple devices, you can install the app on all of your devices without purchasing the app again.
• You can quickly find free or paid apps that you previously downloaded.

App purchases are not bound to devices, but to the Google account they've been purchased with. So as long as you use the same Google account on the devices, there should be no limit AFAIK.