The cards will be tide to a credit card number (or to a persons cash deposit) not to a room number, - for use as the sign and sail card. The purser should be able to re-assign the card to a cabin door for use as a key.
When you fill out your boarding passes, there will be a page that asks for the credit card info per guest, as long as that gets filled out, I don't think that should be a problem. If you plan on changing which cabin each of your nine people are assigned to, then you may have to do that at boarding time.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013