The number of circuits will vary tremendously between individuals. I recall on RAF Flying Scholarships we sent students solo after about 24 circuits; and 9 hours of flying; in many cases this was too soon but the RAF would not let you go beyond 10 hours without individual authorisation. I suspect most PPL candidates are nearer 50 to solo. There is a growing trend for instructors to go into the circuit before all the basics have been taught; this increases the number of circuits and often results in a poor standard of flying.
If the interest in the number of circuits is based upon the cost of each circuit (landing fees not aircraft cost) then it is worrying that safety is being affected by the need to reduce the number.
As a PPL examiner I have noted that the standard of landings is now very poor; under the old UK system where general handling was tested in a one hour GST a much higher standard was observed. The lengthy JAA PPL Skill Test means that candidates seldom give their best and examiners frequently give the tired student the benefit of the doubt. It would appear we need more rather than less circuits.