There is no way in game to see how many people are logged in. The only way we have to judge how popular a server is is how frequently it has character creation or log in restrictions imposed.

Honestly, since the Duty Finder for dungeons etc is cross server anyways, it is likely not to matter until you are at or near the level cap as regardless of server size most people don't chat in the open world but instead in their Free Companies (FC) and Linkshells (LS). If you want lots of people to socialize and play with, I'd suggest finding a large FC and/or LSes. You can have only one Free Company, but several linkshells.

If you don't like your server and want to leave now, I'd just save your character design in the login window and recreate them on another server since you are not very far along, and server transfers cost money.