Doesn't China probably need at least twice the JMSDF number, given that the PLAN needs to patrol both the South and East China Seas?
Not quite that simple -- Japan in terms of geography is surrounded by water on all sides, so in a way there are more vulnerable to naval and undersea threats than countries which are semi land locked, so the JMSDF may technically have a higher requirement for ASW MPAs than the Chinese Navy in terms of patrol area.
It also wouldn't be entirely accurate for us to believe that facing a large number of adversary submarines necessarily means one has to have more ASW MPAs, as if all the adversary submarines are operating within one location that all your ASW MPAs can reach from shore without difficulty and remain on station for a long time, then one can use simply a single ASW MPA to prosecute multiple targets. Conversely, if the opposing submarines are very quiet and unable to be detected by your sensors, then sending in ten times the number of ASW MPAs in one location will not necessarily mean you can prosecute it easier.
The size of the ocean in which one wants to sanitize of submarines is very important in determining the number of ASW MPAs one needs, I think, so it's absolutely poor logic to think that China's required MPA number should be "twice" the JMSDF's given China has less waters than Japan to watch over.
In the medium term, I expect the Chinese Navy to field maybe 60 ASW MPAs with 20 or so for each fleet, which I think seems like a reasonable number and sufficient to provide the Navy a substantial increase in fixed wing ASW capability that allows them to maintain a good presence in their respective areas of responsibility.
In the long term, I expect maybe somewhere around 90 ASW MPAs in total operating from the mainland, and if the Chinese Navy starts to establish overseas bases with their own MPA requirements then it may be viable for them to forward deploy some of them around the world. But for the forseeable future a triple digit ASW MPA fleet is not entirely necessary IMO.
