"insufficient numbers of" is correct and usable in written English.
It is often used when talking about something that is not enough. For example, "There were insufficient numbers of teachers in the school to cover the classes."
There were insufficient numbers of qualified mathematics teachers.
And so far there are insufficient numbers of graduates to force big shifts immediately.
Insufficient numbers of military police units were sent to Iraq in a disorganized fashion, many of them untrained reservists.
"These shortages are compounded by insufficient numbers of young people, especially girls, choosing a career in engineering.
"Minority officers feel that the problem is not numbers," he said, "it is that there are insufficient numbers of senior officers from minority ethnic groups.
10.36am: The Institution of Engineering and Technology has been in touch to warn of a "technology skills gap", with "insufficient" numbers of young people studying such subjects.
Starting in 2000, with insufficient numbers of Japanese job applicants, the chain turned to foreign students who are allowed to work part time.
The Confederation of British Industry (CBI) regularly complains that there are insufficient numbers of people trained in science, technology, engineering and maths (STEM) at all levels.
Elementary education is free and compulsory, by law, but there are insufficient numbers of schools and teachers to fully supply it.
Some researchers attributed the persistent issue to a "pipeline problem," insufficient numbers of senior level women in medicine, particularly in certain specialties like surgery.
There was no recruitment from one correctional centre due to insufficient numbers of Indigenous prisoners.
