WordNet records no synonym and no antonym for modicum. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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 small or moderate or token amount
“England still expects a modicum of eccentricity in its artists”
Synonyms, antonyms and the broader and narrower relations for modicum come from Princeton University’s WordNet. Full sources and licences.