In short, a Beams Subscriber is a device registered to receive Beams notifications from your instance(s).
All push notification gateways provide a secret token for each device (Android device, iOS device, web browser) you want to publish to. When you start the SDK (by calling the `.start` method) the device will register its token with Beams. Each of these devices is a counted as a "subscriber".
We calculate the number of subscribers by taking the sum of concurrent Subscribers across all your Beams instances (for a given day).
You can unregister devices from Beams when they are no longer required to receive notifications. This will reduce your subscriber count and allow you to control your usage. See our guide on this in our Beams docs.