How many are able to be defensible? (3)

I believe the answer is:

ten

I'm a little stuck... Click here to teach me more about this clue!

'how many' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for ten that I've seen before include "Half a score" , "How many" , "Figure" , "PM's address, Downing St" , "Roman numeral X" .)

I've seen this clue in the Evening Standard.

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