this world is not accepting any new characters

[Seleni]

And how do I know when population goes below? As if the game is telling me that.

The page Enla linked in a post above is a status page with (likely real-time status) icons indicating whether the world is allowing new character creation. Green person icon with a + sign indicate that you can create new characters then.

If you don’t like clicking on links, You can get there from Lodestone page: select News - Server Status
Full link: https://na.finalfantasyxiv.com/lodestone/worldstatus/

It’s more likely to change into green icon during off-peak hours.