Several means some, and used with plural countable nouns.

• Several people have complained about the waiter.

• There are several errors in your composition.

• I lost the case for several reasons.

• He bought several books about English grammar.

• I’ve seen ‘Matrix’ several times.

When used with of, it must be used with determiners like (the, my, your, these, those)

• Several of my friends are learning English in London.

• Several of the cars were sold last week.

• He lost several of his books.

• Several of them died in the bomb attack.

• Several of us cannot graduate next year.