https://ffxivcensus.com/ active characters shows how many have completed the 5.0 story quest as of April and it's as close as we've got
I think new actual servers (not housing wards) would definitely alleviate the housing issue a bit, provided people are willing to move. Spriggan and Twintania only recently ran out of plots with ~7k players on them. All the other NA and EU realms have 10k+ characters on them that have finished 5.0. JPs housing problem is bad (no plots available) but it's not as severe as the NA/EU one because there are 32 JP servers vs 24 NA and 12 EU servers.
Alternatively, if we do get Ishgard housing, only a few more wards would be needed even for the largest servers in the game to have their demand pretty much covered for.