simplicity has 7 distinct synonyms spread across 5 of its 5 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 complexity.
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 quality of being simple or uncompounded
“the simplicity of a crystal”
a lack of penetration or subtlety
“they took advantage of her simplicity”
freedom from difficulty or hardship or effort
“he rose through the ranks with apparent ease”
lack of ornamentation
“the room was simply decorated with great restraint”
Synonyms, antonyms and the broader and narrower relations for simplicity come from Princeton University’s WordNet. Full sources and licences.