Hi, Are you referring to this - Azure Service Bus Messaging ?
https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-dead-letter-queues
I don't know too much about the above, but if you do get the count into Flow using an API call or similar, the something like the below can be used to see if it is above or below a value. If true, send an email, if false, don't send.
Could you please as much detail as possible?
Can you post an image of any Flow you hvae srated to create to help illustrate the issue?
What have you done so far to try and "get the count of meesages in the deadletter Topic subscription"?
Is an HTTP request you are trying to use or what method of query?