Well you have the fact that as of Feb 2022, over 800k characters completed the MSQ at least. That's up from previous expansion numbers, which were about 560k for SHB and 616k in SB in the same timeframe.
https://www.reddit.com/r/ffxiv/comme...tails_are_out/
You have a more accurate comparison that Lucky Bancho's site offers every few months:
Heres the report from Janurary:
https://luckybancho.ldblog.jp/archives/56364034.html
Heres the report from April:
https://luckybancho.ldblog.jp/archives/56584369.html
Jan 2022:
https://livedoor.blogimg.jp/luckyban...3/e3af7fb0.png
April 2022:
https://livedoor.blogimg.jp/luckyban...2/d20b09dd.png
Rough Translation of the numbers:
The number of active characters is about 1.7 million, an increase of 350,000 from the previous (1.35 million).
The number of new characters increased by about 250,000 from the previous 180,000 to 430,000.
The last time inactive was 160,000 returning characters that were active this time, up 40,000 from the previous 120,000.
The number of active (return?) characters since the last time has increased from 1.04 million, up 60,000 to about 1.1 million.
The moon started at 1.33 million (up 300,000). The Dawning Level Cap reached 970,000 (up 330,000). The moon clear is 880,000 (up 310,000).
So game is definitely growing. They are mostly parsing data from lodestone against hitting certain quest milestones in MSQ, some side quests, Job levels, etcetc. There's a GitHub that explains how they classify the data a bit more and how they weed out and account for bots.