Duo; a couple (3)
I believe the answer is:
two
I believe this is a double definition.
'duo' is the first definition.
(a duo is two people)
'a couple' is the second definition.
(a couple can mean two of something)
(Other definitions for two that I've seen before include "Pair, deuce" , "Number of Gentlemen of Verona" , "Prime number" , "Company allegedly" , "Old song 'Tea for . . .'" .)