"She knitted me about a hundred sweaters!"
Translation:Elle m'a tricoté une centaine de pulls !
there are two explanations that fit this. take your pick. both are valid. that's the complexity of modern grammar.
in english the expression 'about YYYYY' designating a quantity is an adverb, so 'une cinquantaine' is a noun being used as an adverb. such adverbs just use 'de' to introduce the complement 'pulls'.
'une cinquantaine' is a noun and 'de pulls' is the modifier of that noun; an adjective if you will--'une cinquantaine de pulls' these two phrases form a 'noun of noun' and the modifier (a noun, pulls) becomes an adjective by adding 'de' to it.
in both cases, 'de' is not acting grammatically, it is lexical.
'une' is necessary because these '-aine' nouns can also have a plural form using 'des' (des is the plural of un/une). 'une' has no connection to the modifying term 'de whatever' (here de pulls). whether singular or plural it will always be 'de'--de marbles, de goldfish, de bobby pins. and will always be a plural noun.
"j'ai une cinquantaine de pulls, mais dans l'entrepôt, j'ai vu des cinquantaines de pulls." (i have about a hundred sweaters, but in the warehouse i saw hundreds of sweaters.)