The phrase "pupil numbers" refers to the total count of students in a school or educational institution. It tells how many students are enrolled in that particular place of learning. Full definition
The issue has arisen as a result of the boom in birth rates leading to a rise in pupil numbers in schools and under - recruitment to teaching posts over the previous five years. (educationbusinessuk.net)
That's happening at the same time as rising pupil numbers and problems with teacher supply. (educationbusinessuk.net)
He also said that, because of increasing pupil numbers, the school's budget would still amount to a real terms cut in funding per pupil. (bbc.co.uk)