200 J-20s would probably not be enough, but given China's economic slowdown (and U.S.-China Trade War and economic isolation starting to bite), I am not sure if the PLA could afford several hundred J-20s during the mid. 2020s.
China grew more this year than last year though... and grew more last year than it ever had done before. Looking at wage increases, employment rate etc shows that the economy is booming.
Several hundred J-20 (as in low hundreds) would easily be affordable, unless China cuts the PLA’s budget even further, but I don’t think they’ll do that. Any more budget cuts and China practically becomes a demilitarized country.
By 2025, US would likely maintain 400 F-35 (85 million $) and 200 F-22 (140 million $), using a budget 3 times the size of the Chinese one. However, the Chinese economy is 25% larger and by 2025, it could reasonably be 50% larger, which means US budget would only actually be 1.5 times as large in absolute numbers. PLAAF can easily maintain 300 J-20s at 120 million $ apiece.