Similar to the case of the J-7s, the IISS appears to be late by ~1 year.
They counted 8x operational ABs with J-20s, which at full strength would amount to 240x J-20s. There are also a few tens (?) of J-20s in some test and training units. As such, they arrive at a count of 230+ J-20s, which isn't unreasonable on the basis of their unit count.
Meanwhile, a proper count of J-20 units shows that, in November '24, there were 10x operational ABs fielding the J-20. The IISS appears to have missed the 97th AB (converted in late '23) and - unsurprisingly - the 19th AB (converted in August '24).
Today (i.e., June '25) there are 11x known operational ABs fielding the J-20, which suggests a total of up to 330x J-20s are in operational service, with a further few tens (?) in T&T units. IMO, an estimate of ~350x J-20s in PLAAF service is reasonable, whereas the total produced (but not necessarily inducted - which is what the IISS counts) is certainly above 400x.
Overall, it appears that IISS data on the PLAAF, claimed to be current as of November of a given year (in the latest case, 2024), is in fact more-or-less current as of November of the previous year (in the latest case, 2023). Which is IMO decent for an organization that tracks the fluctuating OoBs and equipment holdings of almost every military in the world without an exclusive focus on PLA watching.
1. Your figures look pretty reasonable!
2. If ~350 J-20s are in PLAAF service even though over 400 airframes have been produced, then what are 50+ J-20s up to?