I'd say were are around 15-16 flying aircraft including 2001 and 2002, I just haven't seen any evidence to contradict that serious "guestimation"?
I don't think we have any good estimates of what the top number of J-20s can be.
Unfortunately they seem to have clamped down on photos of CAC can prevented us from discerning new production airframes with serial numbers.
However, I doubt there are only 15-16 total aircraft flying (i.e.: including 2001, 2001 and the 201X prototypes, as that means there would only be 8 LRIP aircraft).
I personally wouldn't be surprised if they were approaching 20 LRIP airframes (not including prototypes), but I wouldn't stake my name on it.