We have studied Relative Pronoun and the Verb in the previous chapter and ended up with Rule 20. Here, we are going to learn Numeral Expressions And The Verb, continuing from Rule 21.
Numeral Expression and the verb

Let's go ahead.

Rule 21 —

Indefinite Number/Definite Number + of A number of/The number of (a) A number of is Indefinite number. (b) The number of is Definite number. Therefore A number of + Noun always takes the verb in the Plural Number because Indefinite Number is believed to be Plural. The number of + Noun takes Singular Verb because Definite Number is believed to be in the Singular Number. As –

(a)
  1. A number of boys have come.
  2. A number of books have been purchased.
  3. A number of children are playing.
  4. A number of students are absent.
(b)
  1. The number of students is going down.
  2. The number of graduates is increasing.
  3. The number of employees is fixed.
  4. The number of guests varies.
In the same way the following are some more phrases showing Indefinite Number/Definite Number in which the same rule applies –
(many of, a handful of, the rest of, half of, a quarter of, some of, most of, majority of, minority of, part of, percent of, none of, all of, a few of, etc.)