"common to have 18 in the circuit"!!!!! Are you havin' a laugh? Is he havin' a laugh?
If you ask me, 18 aircraft in one circuit is a failure of the controller to maintain a safe airfield. That is ridiculous. What separation would that provide between machines? 18 aircraft?!? Break it down. 4 legs in the circuit. Climb out/crosswind, downwind, base/final and traffic on the runway. 4 machines on each of those segments is far more unsafe than what you would expect to see on an everyday airfield.
18?