Grammatical Number is the term used to describe whether something is singular (just one) or plural (more than one).

In English this applies to nouns, verbs and determiners and depending on the number the form of these may change.

Nouns

The usual way to change a noun from singular to plural is by adding -s or -es.

One chair. Two hundred chairs.
One box. Fifteen boxes.

For a full look at forms of plural and singular nouns, see here.

Verbs

Number is important in verbs as a verb must agree with the subject. If the subject is singular so must the verb be; if the subject is plural, so must the verb be.

He is coming. We are coming.
She doesn’t like it. They don’t like it.

For more on Subject-Verb Agreement see here.

Determiners

Depending on how many we are talking about, determiners can change:

A person. The people.

For more on Determiners see here.