The phrase "decrease in number" is correct and usable in written English.
You can use it to describe a decrease in the quantity or amount of something. For example, "The population of endangered species has seen a significant decrease in number over the past decade."
As manual workers decrease in number, and as all workmen are increasingly educated, the over-all Party membership has lessened in proportion to the growing french population.
They are highly concentrated along the plane of the Galaxy and slowly decrease in number outward from its centre.
Danger to spectators, racers, and livestock on roads not built for the automobile, let alone racing, ultimately caused road races to decrease in number.
When the concentration reached 100 μg·mL−1, apoptosis, necrosis, and decrease in number of cells were observed.
At some point the predator population over eats and the prey population begins to decrease in number.
In most Gbx2 −/− mutants, thalamocortical axons decrease in number and grow no farther than the internal capsule (subpallium).
For example, some species that are sensitive to pollution, will decrease in number drastically or, if too sensitive, may completely disappear.
Furthermore, the extent as well as relative increase or decrease in number of significantly regulated transcripts varied between time points and tissues.
That Coach continues to offer repairs is handy, since independent leather repair specialists are expected to decrease in number over the next decade. .
Two months into Euromaidan, the celebration of Christmas and the "New Ukraine" sculpture were hardly indicators of an end to the protests, although demonstrations began to decrease in number.
As new cells are added to various niches of the immune system all existing sub-populations will decrease in number, making room for the new cells.
