Quantifiers are one of four classes of words (other than adjectives) that can occur before the head of a noun phrase--the others being predeterminers, determiners, and ordinals (cf. 1). There are closed system (i.e. a limited set of items) and open-class (an unlimited set) quantifiers. The former include many, more, 'most, (a) few, fewer, fewest, (a) little, less. least, and several. Open-class quantifiers consist of words like lot(s), plenty, deal, number, quantity, amount, and bit followed by an of phrase whose noun head or complement is also the head of the entire phrase. Although these open-class quantifiers might be construed as nouns themselves, the fact that the of phrase complement is the head of the entire noun phrase is shown by the fact that when such a phrase is the grammatical subject the verb agrees with the of phrase complement:
A number of nasty remarks were/*was made
A lot of these fads die/*dies out rapidly.