The phrase "decreased in number" is correct and usable in written English.
It is used to describe a situation in which the amount of something has decreased. For example, "The population of bees has decreased in number over the past 20 years due to habitat loss."
But lynchings did not stop: they decreased in number, and they went underground.
There are fewer families around since the farms have increased in size and decreased in number.
Today, the breathless articles have decreased in number while the usage has not.
Researchers say the Magellanic penguins at Punta Tombo, which have steadily decreased in number for more than a decade, are not alone.
Citizens Advice bureaus and neighbourhood law centres have decreased in number, and the appetite for the stresses and strains of underfunded and under-recognised work has diminished.
Although the Khanty have decreased in number over the past few centuries, their language is still maintained by about 9,500 speakers (2010 census).
Chinese incursions into Japanese-controlled waters near the Senkaku (Chinese: Diaoyu) Islands in the East China Sea north of Taiwan decreased in number and intensity.
Metropolitan California newspapers have decreased in number, but their total circulation has grown, led by the Los Angeles Times, with the largest number of readers in the state.
Numbers reached their peak before the first world war, at around 64 horses and 48 carts, but after the war, as motorised vehicles became more common, the horses dramatically decreased in number, and had to share their stables with the cars, vans and mechanics.
White storks have decreased in number, but they can still be seen, perched on enormous piles of sticks on chimneys or church towers in areas where unpolluted and undrained marsh is still found.
First, the F-actin puncta decreased in number.
