counterpart vs opposite number

counterpart and opposite number both are nouns.

Word	Noun	Adjective	Verb	Adverb
counterpart	Yes	No	No	No
opposite number	Yes	No	No	No

As nouns, counterpart and opposite number are synonyms defined as:

counterpart and opposite number: a person or thing having the same function or characteristics as another

Other synonyms of counterpart include vis-a-vis.

counterpart (noun)	opposite number (noun)
a duplicate copy	a person or thing having the same function or characteristics as another
a person or thing having the same function or characteristics as another