The most common and most likely answer to this crossword clue is the 6 plus 3 letter word MAGGIE MAY
I've seen this crossword clue in The Guardian
M
A
G
G
I
E
M
A
Y
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 = maggie may.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.