We consider a device to be active if you've asked us to send it one or more notifications within a given period.
Beams billing is based on Monthly Active Devices. For accounts on a Free plan, the most recent 30 days of unique Active Devices will be considered. For accounts on a paid plan, the billing cycle is used instead.
If a user has uninstalled your app, any notifications you try to send to them will not contribute to your Monthly Active Devices.
This information can be found on the dashboard, in the instance page. For example: