7 vertebrae in their necks... read later, phew!
This preserved octopus actually has the normal number of 8 appendages attached to its body, but each one branches out to form the multitude of extra tentacles. Apparently there is no theory that fully explains the surplus tentacles, but they are believed to be the result of abnormal regeneration that occurred after the octopus suffered some sort of injury.
96 armed octopus in Japan display. Regeneration!