Don't think S-E will ever release the number of players per server.
BUT, if you look at this
page from FFXIAH, you may get some ideas of how much more or less populous a server is compared to the next.
(Keep in mind that the "Characters" column denotes
EVERYONE whose transaction has ever been caught by FFXIAH, including all those who have moved on, and does not include those yet to come. Those numbers will continue to grow even as the actual population holds steady around ~500k for all servers.)