Theres a few different types of server status, and the amount of players needed varies by server.

Full/Locked is simultaneous online users.
Congested is based on a sliding scale of total pop, active pop and simultaneous users.

The requirements for those are pretty arbitrary though. JP servers will congest at populations NA servers are preferred and open (at one point JP was congesting on hitting 60% active pop of the lowest congested NA server), and at one time EU servers were waiting until even longer to congest.

Locked,, you just have to character create well outside peak hours.