The most common and most likely answer to this crossword clue is the 5 letter word DOZEN
I've seen this crossword clue in The Guardian
D
O
Z
E
N
More clues leading to the result DOZEN
Buddy explains!

The straight part
We know that "Number" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case number = dozen.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 DOZEN mean?
DOZEN noun- A set of twelve.
- (as plural only, always followed by of) A large, unspecified number of, comfortably estimated in small multiples of twelve, thus generally implied to be significantly more than ten or twelve, but less than perhaps one or two hundred; many.
- An old English measure of ore containing 12 hundredweight.