pleasantness has 1 distinct synonym 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 unpleasantness.
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 feeling caused by agreeable stimuli; one pole of a continuum of states of feeling
the quality of giving pleasure
“he was charmed by the sweetness of her manner”
Synonyms, antonyms and the broader and narrower relations for pleasantness come from Princeton University’s WordNet. Full sources and licences.