A number input is a field for entering a quantity, such as how many tickets to buy. It is made for values that can be treated as numbers, and browsers may show small controls for raising or lowering the value. Think of a kitchen scale: you enter or adjust an amount.
For example, an order form might ask for the number of notebooks, with a minimum of one. A number input can help prevent letters from being accepted as the answer, but the website still needs to check important values when it receives them.
Use it for amounts that can be counted or measured. Do not use it for digit-only details such as a postal code, where the digits are an identifier rather than a quantity.