It is the first time an official figure has been put on the number of people missing since the volcano roared to life, and comes on top of the 47 people whose bodies have been removed from the peak of Mount Ontake.
"The figure is based on information from people searching for the missing," a Nagano prefectural official said, adding that the number may be revised later.
Rescue workers have spoken of up to half a metre (20 inches) of thick, sticky ash smothering the slopes, with some of the dead found half-buried, leading to fears others may be entombed.
Heavy rain forced rescuers to abandon their search of the mountain on Friday, while a gathering typhoon looked set to batter Japan over the coming days, a further reminder of the country's vulnerability to the wrath of nature.
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"We have decided to call off the day's operation as rain continued near the mountain," said another official of the crisis-management office of Nagano.
"We are now also concerned about a typhoon that is forecast to approach the Japanese archipelago and affect large areas, including our region," the official added.
Typhoon Phanfone is predicted to slam into Japan with strong winds and high waves this weekend or later, the meteorological agency said.
Packing gusts of up to 252 kilometres (156 miles) per hour, Phanfone, one of this year's strongest typhoons, moved northwest in the Pacific toward Japan's southwest. It was forecast to make landfall on Sunday.
Autopsies have revealed that hikers, many of whom had been enjoying lunch at the peak in the autumn sunshine, died largely from injuries caused by stones hurled out in the initial explosive eruption at up to 300 kilometres (190 miles) an hour.