Mass production just started cause many things were not finalised for this plane, but no one should doubt China's ability on mass production once it started, China can easily outproduce anyone on this planet.
No one doubts China’s ability to manufacture, especially once the technology has matured. It’s the talk of possibly limiting the production run to 400 J-20s and perhaps 500 J-35s. But considering how mature and capable the J-20 is, a total production run of over 1000 and counting by the middle of the next decade is not impossible to imagine.
Considering the PLAAF still operates the F-7s and F-8s; it would be a sight to behold to see pilots go from those planes to the J-20s. Like that picture of the train operate with the steam train and then the same driver in a high speed train.