CAP670 provides a formula which takes into account the number of positions, hours the position will be available, breaks etc to provide a minimum number of ATCOs; but not support staff
(ATCAs). That is at the discretion of the ATS provider at the unit.
Perhaps there were staff shortages due to sickness, people away on courses, no extra staff at all on that particular day you visited. The ATCO will still cope although traffic may be delayed, safety is inherently inefficient. Welcome to the world of ATC....
