The difference between ‘A number of’ and ‘The number of’

‘A number of’ plus other words will form a plural subject. The following examples will illustrate this fact.

Example 1
A number of plants in this garden were imported from other countries.

Example 2
A number of beggars are usually seen in front of the temple asking for alms.

As a subject, ‘The number of’ with other words is singular. This can be shown by the sentences below.

Sentence 1 : The number of pupils in this class is 30.
Sentence 2: The number of tickets left for sale is 51.

Mistakes occur if other words are added after the number 30 or 51. For example, ‘The number of pupils in this class is 30 people.’ is wrong because you are required to give a number only. Likewise, it will be incorrect to write ‘The number of tickets left for sale is 51 tickets.’ for the same reason.