There are small pools or RPers on nearly all servers, but if you want more than just a small pool of RPers then your choices are either Balmung or Mateus in the Aether datacenter, or Omega in the Chaos datacenter.

All servers occasionally get closed temporarily for character creation during prime time. For most of them it is just a few hours at most, with the rest of the day being fine for character creation. Mateus is no exception to this.

Balmung used to be an exception to this, in that the "temporary" closing used to be active 99.99% of the time - until it was made a "Congested" server at which point it was straight out impossible to make new characters there.
Right now Balmung is open for character creation, but no idea how long that will last.

If you want an RP server in either of the US data centers, go to Mateus. Except for Balmung, no other server comes close to the amount of RP available on Mateus.
Character creation might be closed during peak hours, but during off-peak hours it should be no problem making a character there.