To find out the number of view count on the notifications you’ve sent to subscribers you need to first login with your registered account.

You will be then 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, sent count and view count.

You can also 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.