twenty-one vs 21

twenty-one and 21 both are nouns.

Word	Noun	Adjective	Verb	Adverb
twenty-one	Yes	Yes	No	No
21	Yes	Yes	No	No

As adjectives, twenty-one and 21 are synonyms defined as:

Other synonyms of twenty-one include xxi.

As nouns, twenty-one and 21 are synonyms defined as:

Other synonyms of twenty-one include XXI.

twenty-one (noun)	21 (noun)
a gambling game using cards; the object is to hold cards having a higher count than those dealt to the banker up to but not exceeding 21	the cardinal number that is the sum of twenty and one
the cardinal number that is the sum of twenty and one

twenty-one (adjective)	21 (adjective)
being one more than twenty	being one more than twenty