Hello, fellas. In this session we will learn how to use of in expressions of quantity.
Of is always contained in several expressions of quantity:
a lot of
a number of
a great deal of
a majority of
1) A number of books have been sold.
2) A number of my books have been sold.
Some expressions of quantity sometimes contain of and sometimes not. They use of when the noun is specific or preceded by any possessive, this/that/these/those, or the. Of is not used if the noun is nonspecific.
almost all (of)
a few (of)
a little (of)
one (of), two (of), three (of), etc
1) Many of my students are foreigners.
2) Many of those students are foreigners.
3) Many of the students are foreigners.
4) Many students are foreigners.
Betty Schrampfer Azar, Understanding and Using English Grammar: Third Edition