Which version of the stack are you in?
You could create an index threshold alert in Kibana that would trigger if the number of documents in your index is lower than some specific value for a specific time interval.
If you are running with the basic license the only actions for this alert are to index the alert in some index or log it in the Kibana log file, but you could index the alert in some index and then use a open source tool like elastalert2 to query this index and send an e-mail.