A number option type allows a user to input a number.
A number input field will also create a new calculation variable that allows you to get the user input value and use it in a formula. This can be used in the number formula field itself, another options formula or an option set calculation field.
Creating a new number input with the unique name quantity will produce a calculation variable quantity and quantity.inputvalue. These variables can be used in any calculation field.
Special trick. You can set the value of a number field to be the user input value by setting the number field to use a formula and setting the formula to the calculation variable for the .inputvalue of the number option.