Background: Devices can in an a messaging topic, hence pus/sub applieds for some devices.

So one message to the topic can be received on an unknown number of devices.

Legacy devices are not able to join a messaging topic, so those are explictly known.

This may lead to a weird number of receivers 21 (e.g. 20 legcy devices, 1 topic (likley able to address tons of devices).

Not sure how to best word it.

Background on Firebase Topics, is here:
https://firebase.google.com/docs/cloud-messaging/android/topic-messaging