I had a think about the number of airframes at Thorney; I guess what Andy's book gave, 6, was the initial delivery.
This would have been sufficient for the OCU staff to get to grips with them while No1 and then 2 Course were in ground school. Once the students started flying 6 would not have been enough and a few more would have been delivered - hence why I flew in 9 in the 10 week flying stage.