Any number of players above 500 is fine. Blizz servers where originally capped in the region of 1000-1500 players, only the servers that got cues in peak hours ever hit those numbers.
Until nost did a heavy revamp on number of resource nodes and re-spawn rates (going dynamic) the world grinding was very overcrowded at "only" 4k.
Ofc keep an eye on those server numbers if they keep going down past the novelty of the war effort then there could be a problem, however transfers aren't the solution.
When blizz opened paid transfers there where several servers who more or less died while others got so overcrowded that the players already there got frustrated and it turned into general internet flaming between the groups. This was mostly because everyone went to the servers with the "best" raid guilds (the most popular and visible).
In the case of free transfers it would be the same as shutting down the server since everyone wants to be on the "winning" side.
On the topic of players and guilds leaving. I wouldn't be worried unless there was a MAJOR drop in players as it happened back in the day as well, priorities change, rl happens and on occasion ppl just grow bored of the game or even a new game launch which sucks up activity for a time. It just gets so much more attention when it happens to prolific guilds.
The concept of waiting for months at a time for new content was the norm. With 300+ (counting low as not all slots has things of interest in all 40 man content) item slots to fill in your raid and only so much loot/reset you really need months at a time. Though I do agree that the war effort requirements though well intended turned out a bit harsh.