limitless has 6 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.
without limits in extent or size or quantity
“limitless vastness of our solar system”
having no limits in range or scope
“to start with a theory of unlimited freedom is to end up with unlimited despotism”
seemingly boundless in amount, number, degree, or especially extent
“unbounded enthusiasm”
Synonyms, antonyms and the broader and narrower relations for limitless come from Princeton University’s WordNet. Full sources and licences.