Set default "Quantity" dropdown select in product. Commerce Kickstart
I know its already a year ago, but if someone bump in this question here's one of the solution, I don't know if there's a module that already existing changing the widget type of quantity in add to cart form but you can achieve this using
hook_form_commerce_cart_add_to_cart_form_alter like this

Example:

function example_form_commerce_cart_add_to_cart_form_alter(&amp;$form, &amp;$form_state) {
  // Change widget type of quantity to select list
  $form['quantity'] = array(
    '#title' =&gt; t('Quantity'),
    '#type' =&gt; 'select',
    '#datatype' =&gt; 'integer', // &lt;-- I added this one to fix Notice: Undefined index: #datatype in commerce_cart_add_to_cart_form_validate() when you click add to cart button
    '#options' =&gt; array(
      1 =&gt; '1',
      2 =&gt; '2',
      3 =&gt; '3',
      4 =&gt; '4',
      5 =&gt; '5'
    )
  );
}