couple has 21 distinct synonyms spread across 7 of its 9 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is uncouple.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a pair who associate with one another
“the engaged couple”
a pair of people who live together
“a married couple from Chicago”
a small indefinite number
“he's coming for a couple of days”
Synonyms, antonyms and the broader and narrower relations for couple come from Princeton University’s WordNet. Full sources and licences.
form a pair or pairs
“The two old friends paired off”