We are sorry for the confusion. Sorry but GroupDocs.Viewer API doesn't have email notifications when operations limit is reached. The documentation refers to this notification system - http://prntscr.com/4n1iyl . With this system you will be able to check the current states and how much requests in total you have for your subscription plan.
If you need an email notification you can create a custom notification system which will compare total count of allowed requests with current requests count and send email notification.
You can get all information regarding to your operating limits by this method "GetSubscriptionPlanUsage". You can check this method online here - https://api.groupdocs.com/v2.0/spec/#!/system/GetSubscriptionPlanUsage_GET
In the response you will have such object - http://prntscr.com/4n1l7h . As you can see we have a "subscription_plan" - total allowed requests
and "usage" - current number of requests.
After you get this info just compare "usage" and "subscription_plan" values and send email notification if needed.