much has 7 distinct synonyms spread across 3 of its 7 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 little.
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.
to a very great degree or extent
“I feel a lot better”
(degree adverb used before a noun phrase) for all practical purposes but not completely
“much the same thing happened every time”
a great amount or extent
“they did much for humanity”
(quantifier used with mass nouns) great in quantity or degree or extent
“not much rain”
2 of the 7 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for much.
Synonyms, antonyms and the broader and narrower relations for much come from Princeton University’s WordNet. Full sources and licences.