DMARC Analytics shows the approximate number of emails you sent per day. You can choose to show the data for the last day and see the total number of emails for the domain in the report. That is the approximate number of emails sent from the domain because DMARC reports are created for each sent message.
The number is approximate because not all mailbox providers send DMARC reports and a number of messages may not be counted. And this data is coming with a delay because DMARC reports start coming after the messages are sent and happen one time per day.