Can I add a quantity input to the product detail page?

Check out the ProductQuantity project in the Active Commerce Training repository, which provides an example of doing just that. In this case, it uses a text input element, but this could certainly also be a select element to limit the quantity selection.

If you haven't already done so, be sure to check out the other examples located here. This is a good starting point for most common integration points that may be helpful in implementing Active Commerce.