I don't see J-20 entering true mass production until WS-15 is ready. That said, how many F-35s will be stationed around China by the end of 2019?
Considering LM is delivering planes from Lot 9 now, and there have been some 240+ delivered in total, of which some 200+ to US, by the end of 2019 there should be Lots 10 and 11 delivered. That's some 350 F-35 for US, witch each consecutive year 80-90 additional ones (lot 12 is going to be first full production lot)
How many will be stationed depends entirely on the situation. In peacetime, probably 10-50%. In wartime against China, probably 50-80% (since some need to stay back home for training. Lockheed Martin performed maintenance should not be applicable yet since they're all new planes)
As years pass, peacetime percentage is likely to decrease but wartime percentage is likely to remain similar.
Anyway, F35 production schedule is more or less known. Question is, will J20 production rate ever come close? And if so - when?
Producing J16 and J10 is all nice and good - but those aren't planes that will do as well in offensive operations, going out there near the first island chain, let alone beyond it.