To find out the number of notifications you’ve sent to subscribers this month first log in using your ID and password then you will be directed to the dashboard. 

On the left side(vertical navbar) click on Notification > Sent Notification.

You will then find the number of notifications you’ve sent to users.

Click on Export Data. You will then get a file which has a record of a number of notifications you’ve sent, view count, date time etc. There you can add filters and see the number of notification sent in a particular month.