Thanks Marcus,
A few words of praise always guarantees a quick reply

If I understand correctly, your user is looking at the product details and wants to add the product to the cart, but to set the quantity in the product details page.
The Struts action that gets called is the is the AddToCartSubmitAction . If you take a look at the source code of that action class, you'll see that the quantity is hard coded to 1. When a user clicks the AddToCart button in the product details window, a form is submitted (the AddToCartForm.java) which is used by AddToCartSubmitAction to pick out the attributes that were submitted.
What we need to do is to add a quantity attribute to the AddToCartForm. Set the quantity from the ProductDetailsBody.jsp and pick it up in the struts action. This is a relatively minor customization that we could do for you and just ship you the 3 modified files OR if you are comfortable with the explantion and compiling Java etc. you could try yourself. I think you just have one missing bit (the source code of the AddToCartForm which I can send to you).
Let me know how you would like to proceed.
Thanks,
Paolo