Previously we learned how to create a product catalog.
Now, lets customize the Add to cart form a little bit. We will learn how to add quantity field in that form.
Go to admin/commerce/config/order-item-types/default/edit/form-display/add_to_cart.
Drag the Quantity field, and Save the form.
Go ahead and refresh the /products page.
And voila!! You can now choose quantity while adding products to cart.