sustenance has 15 distinct synonyms spread across 3 of its 3 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.
the financial means whereby one lives
“each child was expected to pay for their keep”
the act of sustaining life by food or providing a means of subsistence
“they were in want of sustenance”
Synonyms, antonyms and the broader and narrower relations for sustenance come from Princeton University’s WordNet. Full sources and licences.