Is there a way to set the Qty number from one to zero on the product listing page?
The Ajax Cart uses what you have set on the product - Minimum cart quantity.
This way if you set it to 3 the Ajax Cart button will show 3 by default as the user can't order less than 3 items from this product. You can change this to 0 but I am not sure if nopCommerce can handle such scenarios if the user clicks the Add to cart button.
If you have a grouped product then the Ajax Cart will use the Minimum cart quantity of the first associated product.