There is always possibility USAF cannibalize existing JSDF air base to put high hundreds of F-35 and NGAD to counter J-20. I think sufficient J-20 to utterly overwhelm Japan and keep up pressure elsewhere in 2030s needs to be in thousand range.
Yeah, and I'm sure china already has all the coordinates for every single one of Japan's military bases and big runways near air/military bases, with missiles all pre aimed and zeroed on those bases.
Too early to speculate. China should just keep building up to at least like 6-700 in like 5-10 years time and then see how good drones and ai can get. Also while continuing to build j-11, j-16, j-31/j-35 and j10c and later variants. Keep upgrading and building all of them. Variety is good, just one plane is putting all your eggs in one basket and will make you easier to counter. Even usa knows it can't just mass f35's, since they are too expensive to use and maintain for normal daily regular use. It's like going to local shop down the street in your Ferrari.
Maybe china can develop an even stealthier design that works purely as a command and control center for drones. And is a missile truck that can shoot long range missiles using drones networked targeting info. Ie chinas long range stealth bomber. Those would probably be better if you wanna bomb Japan.