https://na.finalfantasyxiv.com/lodes.../housing_land/
So yes, it is against the rules if they were acquired after the 4.2 changes.Players may only own one private and one free company estate per World on any given service account.
* Players possessing housing above this limit prior to patch 4.2 will not be affected by this change.
The problem with relying on GM responses is that GMs don't create the rules. The rules are created by the developers. The GMs are there to assist players having issues with the game, and given limited authority to enforce some of the rules. Other rules are outside of their authority and related issues have to be escalated to those who handle them. The one GM response I've ever seen posted anywhere regarding multiple house ownership rules said it was outside of their authority to enforce.
Any GM response that goes "well if the game lets you do it, it's okay" is patently false. There have been plenty of things the game has let players do just to have SE state doing so was an exploit and actions will be taken against accounts found to be knowingly taking advantage of the exploit (see Ungarmax as a prime example).
Players still need to use common sense - does the game intend for me to be doing this? In the case of multiple FC ownership, the answer is clearly no. The posted rules say we're limited to one FC house per account per world.
What we're running into is a lack of current enforcement of the posted rules, not a rule that doesn't exist. Let's revisit Ungarmax - that exploit had been getting used for players for a year before SE stepped in, fixed it and started banning accounts.
As for the number of houses needed to meet demand, you're correct. Another 500 houses would not be enough on most NA worlds. But it would still be a start until such time as more houses are added.