The most common and most likely answer to this crossword clue is the 10 letter word POPULATION
I've seen this crossword clue in The Guardian
P
O
P
U
L
A
T
I
O
N
Buddy explains!

The straight part
We know that "Number of people" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case number of people = population.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does POPULATION mean?
POPULATION noun- The people living within a political or geographical boundary.
- (by extension) The people with a given characteristic.
- A count of the number of residents within a political or geographical boundary such as a town, a nation or the world.
- A collection of organisms of a particular species, sharing a particular characteristic of interest, most often that of living in a given area.
- A group of units (persons, objects, or other items) enumerated in a census or from which a sample is drawn.
- The act of filling initially empty items in a collection.