How to Count Mailboxes/Domains?

Domains

In the Admin Level Control panel, select General > Domains Overview. The Overview lists all domains assigned to the Admin account logged in.

The number of Domains assigned to the account is shown at the top of the page:

On the Admin Level Control Panel, navigate to Reporting > Usage Report
The default query rules will show the usage report for the previous 1 day, which can be adjusted to the desired timeframe as needed, and for all domains assigned to the Admin user. This can also be adjusted to show only specific domains using the query rules
See Run Custom Log Search for full details on how to run a custom log search using query rules

The Protected Mailboxes value shows the number of billable mailboxes for the domain in the specified timeframe given in the query rules.

The Archived Mailboxes value shows the number of billable archived mailboxes for the domain in the specified timeframe given in the query rules.

To access the Detected Usage for the Domains assigned to the logged in Admin:

On the Admin Level Control Panel, navigate to Reporting > Usage Report
At the top of the page, click Detected Usage to be taken to the Detected Usage page
The default view for the Detected usage is for all domains assigned to the Admin user logged in, this can be adjusted to show only specific domains or mailboxes using the query rules
See Run Custom Log Search for full details on how to run a custom log search using query rules