Grammarbook.com gave the following rules that will help us learn how to use “The Number” and “A Number”
The expression the number is followed by a singular verb while the expression a number is followed by a plural verb.
- “A number” is always plural.
- “The number” is always singular.
The number of people we need to hire is thirteen.
A number of people have written in about this subject.
Source: Grammar Book
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