Grammatical Number is the term used to describe whether something is singular (just one) or plural (more than one).
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.
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.
Depending on how many we are talking about, determiners can change:
A person. The people.
For more on Determiners see here.Image © Steve took it