The phrase "a decrease in numbers" is correct and usable in written English.
It can be used when discussing a reduction in quantity or amount, often in contexts such as statistics, demographics, or resource management.
Example: "The recent report indicated a decrease in numbers of endangered species in the region."
Alternatives: "a reduction in figures" or "a decline in quantity."
As a result, there was a decrease in numbers of invasive amniocentesis tests that had to be used on the mothers involved in the trials.
Visitor numbers at Britain's museums, galleries, zoos, castles and country houses increased by nearly 9% last year despite a decrease in numbers of overseas tourists.
Although the trend of highway user crashes with rail equipment is showing a decrease in numbers, much has yet to be done to improve the safety of HRGCs.
Note that a large number of extremists from all over the world have answered this call and entered Iraq and Syria to join Da'esh, although there is a decrease in numbers of those joining them now (Foreign Affairs 2016).
The message for a decrease in numbers, even in this nation's worst recession in decades, is simple: "It worked".
The number of Cypriot-born people in Great Britain fell from 78,191 in 1991 to 77,156 in 2001, one of the few country-of-birth groups to experience a decrease in numbers.
Anemia can occur, along with a decrease in number of the white blood cells that are necessary to prevent infections.
Including the higher count would result in an increase from 15 to 16 teeth, followed by a decrease in number with increasing size, as seen in their data.
Using stereology, we showed a decrease in number of GABAergic and cholinergic interneurons in postmortem basal ganglia tissue from subjects with Tourette.
Furthermore, PTH protects bone marrow mesenchymal stem cells from radiation-induced damage, including a decrease in number and an increase in adipogenic differentiation.
When cows are stressed, cortisol secretion increases and immune function is repressed, including the atrophy of immune organs and a decrease in number of lymphocytes and acidophilic leukocytes in blood.
