A key number (6-3)
I believe the answer is:
twenty-one
This clue could be a double definition.
'a key' is the first definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'number' is the second definition.
(I have seen 'A number' mean 'twenty one' so perhaps 'number' could also mean 'twenty one')
Can you help me to learn more?
(Other definitions for twenty-one that I've seen before include "Target total in blackjack or pontoon" , "XXI - used to be the coming of age year" , "Card game, blackjack or pontoon" .)