II don’t thank there is counter like this, it is possible to ser audit log to be decrypted and count established sessions from logs (maybe). I don’’t know about other options. Check self-monitoring as well.
In 7.2 there is a self monitoring dashboard named "Client Communication Monitoring". In older versions there is a dashlet on one of the health dashboards that charts the "Number of Clients" measure that counts the number of fat clients currently connected.