Right now, in the North and south american regions, and Japan (I am unsure about Europe and everywhere else as I don't have access to know these things)
The code is not limited to a certain amount of people, but may only be redeemed once per cartridge.
The code Pokemon497 will get you the newly released contrary Serperior!
The natures are random, but with enough soft resetting you should get one you like
This will NOT work with pokemon X and Y
Happy grabbing Codex.
Edited by Aria, 21 January 2015 - 07:44 PM.