Well... its one key for each disks. In your case you put the same for each disks so its not the default behaviour.
A method would be to:
Early-mount an encrypted disk => you have to give its key.
In this partition (usbkey, whatever) lies the key for the 12 (or 11 disks if you don't have anyother. It can be kept as a file.
Then the second phase would decrypt the remaining partitions using this file as a key.
cryptsetup works in 2 phases for this purpose, which is pretty standard.
Try to have a look at manpages for cryptsetup, /etc/crypttab.
Also /etc/init.d/cryptdisks and /etc/init.d/cryptdisks-early.
These files are from a package called cryptsetup which is a higher application to handle this easier.
Ah what I say probably only works for LUKS encryption.
An other example
# Encrypted luks disk with a openssl-encrypted keyfile, replace with USB Stick
#cdisk4 /dev/hda2 /mnt/hda3/keyfile luks,ssl
cdisk4 will be decrypted using /mnt/hda3/keyfile.
/mnt/hda3/keyfile is the encrypted key file. You provide the password for decoding the key file only. /mnt/hda3 can be your /root or whatever, it just has to be mounted before dm-crypt tries to mount the 12 partitions.
Hope its more or less clear