opposite number vs counterpart

opposite number and counterpart both are nouns.

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

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

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

Other synonyms of opposite number include vis-a-vis.

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