To implement a quantity field in a calculator, just use a number input element from the list of elements
In the properties of the number input element, just enable the toggle called "Use as Quantity"
Scroll down to see the calculator configuration.
In the Formula Configuration, don't include the quantity.
In the Formula Label, please put the price per item.
Lastly, if you want to show the total price on the product page, you can have the calculation display element.
Multiply the quantity in the Formula. Just like the image below.
Here is the output once the calculator is applied to the product page
Here is what you can see in the shopping cart.