There is no detail information on the nymber of JL9 in service, but we can use information from Huitong's Chinese Military Aviation to deduct a conservative minimum numbers.
The followings are the regiment/brigade/aviation university fleet serial numbers, assuming they operate 8 aircraft each (conservative minimum) :-
83X71 83X5X 78X2X 78X4X
4X8X 1X3X 3X2X 2X4X 1X1X
9 rejiment or brigade x 8 aircraft = 72 (assuming 8 aircraft per regiment/brigade etc)
If you go for the high end number of 24 aircraft per regiment/brigade, the number would be 9 X 24 = 216 aircraft.
However, some training regiment have a mixed fleet of JL9 and JJ7, so the real number is probably around 100 to 150 aircraft.