singly has 5 distinct synonyms spread across 1 of its 2 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 multiply.
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.
one by one; one at a time
“they were arranged singly”
apart from others
“taken individually, the rooms were, in fact, square”
Synonyms, antonyms and the broader and narrower relations for singly come from Princeton University’s WordNet. Full sources and licences.