You would need to manually update this in the code, it would require you to add in or alter the if statement so that it was looking at the posted quantity for that product. The if statement might look something like this:
if(isset($_POST['Quantity_1']) && $_POST['Quantity_1'] > 0){
// add to cart server behavior code here
This is a bit custom though and not something that we support. I would suggest looking for some resources on php and and processing post data to get more info on how these things should be crafted.