A single client can subscribe to many different destinations
over a single channel. Also, a single client may have more than one
connection to the server (say its using an AMF channel for
RemoteObject calls and an RTMP channel for subscriptions although
you should always try to use just a single channel on the client
for everything). So there's no relationship between a
channel/endpoint and subscriptions.
Are you interested in a count of total subscribers for a
destination or across a set of destinations?