The new estimate follows reception from the Greek authorities of a definitive list of who was on board the boat, which was travelling from Greece to northeastern Italy when it caught fire before dawn on December 28th.
On the basis of that, there are 16 paying passengers unaccounted for, along with two Syrian men reported by their families to have boarded as stowaways.
Italian authorities said in the immediate aftermath of the rescue operation that nine bodies had been recovered from the blaze-hit boat or the waters around it. It was unclear if two others identified as dead in the water at the time were counted among the 16 passengers listed as missing on Thursday.
The ferry disaster also claimed the lives of two Albanian seamen killed in a tugboat accident during the rescue operation in stormy seas.
Rescue services took more than 24 hours to get survivors off the blaze-hit boat, which was towed to the Italian port of Brindisi last week.
Fears many stowaways may have perished were ignited after two Afghans and a Syrian were rescued alive. They have applied for political asylum.
The cause of the onboard fire remains unknown.