I have all the stats in a giant table going back to 2014.
- ARR's average was 600,000 (the fluctuation was 574,440 to 670,169).
- Heavensward's average was 600,000 (the fluctuation was 824,556 to 496,412 in what was a progressive decline).
- Stormblood's average was 600,000 (the fluctuation was 536,830 to 831,334).
- Shadowbringers' average was 900,000 (the fluctuation was 862,032 to 1,254,294).
- Endwalker's average was 1.2 to 1.3 mil (the fluctuation was 1,115,937 to 1,702,744).
(This is ignoring the 1,624,238 which happened during a patch lull when the game went viral, because it was not reflective of the population that stuck.)
All the "fluctuations" were obviously due to patch lulls and the fact the population usually slides down as the expansion progresses, due to there being less hype than at the expansion release. However, there are usually spikes sometimes except in Heavensward.
Although I can't say for sure, comparing a number of spikes makes it seem unlikely it'll drop below Shadowbringers levels, but it is likely it could be at the Shadowbringers average (900,000).