So while there's 300k plots world wide (293,760 to be exact), that means that for me in NA, there's only 103,680 available for the entire region, but as I'm on Aether, that means that the number is actually 34560 for the datacenter... which means that I'd have to go offworld to get to my house (I'm not sure that's even supported), so for Cactuar it's only 4320 houses (4 neighborhoods * 18 wards * 60 houses).
Going by
https://ffxivcensus.com/ , there's almost 16M created characters, with about 750k considered "active" across the world by their definition of "active" - which is basically endgame characters (and ignores the leveling ones). For Cactuar that's 16k "active" players... vying for those 4k houses which also completely ignores the FCs. Oh, and Gilgamesh has about 17k active members on it, so your greed is actively making a bad situation worse on your home server, as you are denying about 20 other players a home going by your signature.
So while we have the apartments, the reason why I'm not counting those is that they do not have access to many of the features of the FC houses (like the workshop and gardening), so for the scope of what's driving this issue the apartments don't matter - despite adding a fairly significant number of domiciles for purchase.