every time someone says "a chip shortage, or they can't find more servers".

[Hustensaft]

And until recently, the EU likely was fine for server space. Somebody suggested before that when more EU servers were added that people complain about how long Duty Finder queues were, so there is a disadvantage to having too many servers. So I would suspect JP has the number of servers to match their population else I suspect they'd run into low population complaints.

NA and EU Worlds haven't been fine since Shadowbringers launch.

There are several people tracking the active characters on the lodestone. In these benchmarks, EU and NA have been at over 20k active characters per world since June 2019. In June 2020 they were at 22-24k. All while japanese servers were at 11k per World. JP just has vastly more worlds per DC and per active player. (let's leave the housing implications per world out for now)

Population grew slowly but steadily and servers have been full. They just didn't do anything and had no plans to do so until post Endwalker launch. It's a massive failure on their planning and managment department since they cheapened out to future proof anything.