Select “Custom Forms” from the left side Menu and Click on “Forms” button

On “Custom Forms” page, click on “+Add New Form” to create new form

On the Form page, enter Form Name and click on the radio button to choose the type of form – Product Related 

Select Header Icon and Product Icon from the dropdown list and choose the group and users

Choose the categories to show the form and click on "Save" button

Now, click on more icon at the right side of name field

You see “Add new customer field” and “Add New Product field” sections as shown in the image above
Now, choose the “quantity” under “product field” and click on pencil icon to edit it

Tick on the check box of “Show quantity on product page” field and click on "Save" button

On web catalog, users can see the quantity of the product items on “Order Cart” page