I can only speak for myself.Wikipedia states it. I thought therefore 28 was the number veteran members had in their minds whenever a discussion on the actual number of j-20 took place. Counting the serial numbers (as another member pointed out) would only decrease the total produced.
A much more precise figure would be appreciated.
In terms of J-20 in service numbers there are two "categories" that I think we can work with.
(I'm leaving out the J-20 prototypes; 2001, 2002, 2011, 2012, 2013, 2015, 2016, 2017; they are prototype aircraft and not production aircraft so are not included)
One category is "confirmed serials" -- i.e.: aircraft whose serial numbers we have positively identified with pictures. Of the J-20s with confirmed serials, they include 78271, 78272, 78273, 78274, 78275, 78276, 78277, 78278 as well as 78230, 78231, 78232, 78233, and 62001. Those aircraft are confirmed serials from three units and they total 13 "confirmed" in service/production aircraft.
The "confirmed serials" form the minimum bedrock of J-20 numbers that we know are in service.
The other major category is "actual J-20s" -- i.e.: it is made up of the number of J-20s in the "confirmed serials" + "X".
X in this case = the number of J-20s that may exist in service but whose serial numbers we have been unable to identify or which have not been released.
The problem is we have no idea how big "X" may be at any one time. For example, we received the photo of J-20 s/n 62001 deployed at 9th brigade in the last few days, however does that mean there is only 1 J-20 in service at 9th brigade at present when the photo was released? Probably not -- here is a satellite photo taken early in April of 9th brigade at Wuhu, showing three J-20s peeking outside of their shelters. We had photos and rumours that 9th brigade had begun receiving J-20s much earlier in the year as well, so the question we are left with is how many J-20s does 9th brigade actually have at this time?
The same question is then extrapolated to other units that have received J-20s -- i.e.: how many J-20s does 172nd and 176th brigade actually each have, on top of the confirmed serial numbers?
It is also very difficult for us to estimate how many J-20s CAC may be producing annually, because:
1: we don't know if J-20 production at CAC is done in an annual manner or not
2, and more importantly: we don't have any semi regular or semi up to date pictures of CAC taken from the ground that shows J-20s with identifiable numbers that allow us to estimate how many J-20s have been produced or how many J-20s may be produced in each "batch" or "group" or what not.
Therefore, as of today, the number of "actual J-20s" in service (or that have been produced) is the 13 J-20s of the "confirmed serials" group, plus however many J-20s exist in the group "X".