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.
Feel free to search our site for a full look at forms of plural and singular nouns.
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.
Feel free to search our site for more on Subject-Verb Agreement.
Determiners
Depending on how many we are talking about, determiners can change:
A person. The people.
Feel free to search our site for more on Determiners.