swarm has 7 distinct synonyms spread across 4 of its 4 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 group of many things in the air or on the ground
“a swarm of insects obscured the light”
Synonyms, antonyms and the broader and narrower relations for swarm come from Princeton University’s WordNet. Full sources and licences.