To sign up for beta test it is required to have a product of activated and your Battle.net account in order to select the games you want to test. If you choose this one of the rare cases when there is no need to be bound by the "contract of silence", but the contents will be available in beta are still subject to change and may be drastically different in the final edition.
In this press release Blizzard said to players to be ready for phishing scams, so better check on whether you apply or not to your Battle.net account.
For the number of participants in the beta charge explained that initial goal will be to allow access to test site interested annual subscribers, and if necessary will invite additional players to the beta test. The duration is still unknown, and although no exact date for the start of the beta was announced, that will happen soon.
It is always exciting to start a new expansion after being in the game for so long, and completed all the instances, got the best gear for PVP so it will be a great proposition to start the new expansion with the batteries charged. All in all The Mist of Pandaria is expected to be a great overall and to return the old subscribers back on track.