number has 18 distinct synonyms spread across 10 of its 18 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 property possessed by a sum or total or indefinite quantity of units or individuals
“the number of parameters is small”
a concept of quantity involving zero and units
“every number has a unique position in the sequence”
a short performance that is part of a longer program
“he did his act three times every evening”
the number is used in calling a particular telephone
“he has an unlisted number”
one of a series published periodically
“she found an old issue of the magazine in her dentist's waiting room”
a select company of people
“I hope to become one of their number before I die”
a numeral or string of numerals that is used for identification and may be attached to accounts, memberships, etc.
“she refused to give them her Social Security number”
a clothing measurement
“a number 13 shoe”
a numbered item in a series
“take the number 2 to the main square, then change to the number 5”
the grammatical category for the forms of nouns and pronouns and verbs that are used depending on the number of entities involved (singular or dual or plural)
“in English the subject and the verb must agree in number”
an item of clothing
“she preferred the black nylon number”
Synonyms, antonyms and the broader and narrower relations for number come from Princeton University’s WordNet. Full sources and licences.