Is there a proprietary way in rabbitMQ(configurable) to automatically notify our DL after crossing few number of message in dead lettered exchange via some plugins(management plugins).
There isn't anything built in to RabbitMQ to handle this. But if you have any monitoring systems / solutions for your network already, there is likely a plugin for what you are using.
Check out the list of management / monitoring plugins on the site: http://www.rabbitmq.com/how.html#management