One needs a couple of rings to get this number (7)
I believe the answer is:
hundred
'this number' is the definition.
(hundred is a kind of whole number)
'one needs a couple of rings' is the wordplay.
I cannot quite see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found within the answer.
This explanation may well be incorrect...
'to get' acts as a link.
Can you help me to learn more?
(Other definitions for hundred that I've seen before include "Number; county subdivision" , "Old part of county" , "cricketing achievement" , "part of Chilterns maybe" , "Former division of county" .)