AISHK waiting list

My son applied P4 this year, we received the letter from AISHK said that my son had met all necessary requirements but they can't give us a formal acceptance due to the space limitation, they will put us in the wait pool and will be in contact with us once they find a space permits.
Has anyone received the same letter with us How big the chance to be accepted at the end?Or is there anyone encountered the same case last year? Have you get in finally? Can anyone please share your experiences? Many thanks in advance!