The most common and most likely answer to this crossword clue is the 8 letter word QUANTITY
I've seen this crossword clue in The Daily Telegraph
Q
U
A
N
T
I
T
Y
More clues leading to the result QUANTITY
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 = quantity.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 QUANTITY mean?
QUANTITY noun- A fundamental, generic term used when referring to the measurement (count, amount) of a scalar, vector, number of items or to some other way of denominating the value of a collection or group of items.
- An indefinite amount of something.
- A specific measured amount.
- A considerable measure or amount.
- Property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as number and a reference.
- Indicates that the entire preceding expression is henceforth considered a single object.