Open modules/ubercart/uc_product/uc_product.module

Around line #1299 replace

  if ($node->default_qty > 0 && variable_get('uc_product_add_to_cart_qty', FALSE)) {
    $form['qty'] = array('#type' => 'textfield',
      '#title' => t('Quantity'),
      '#default_value' => $node->default_qty,
      '#size' => 5,
      '#maxlength' => 6,
    );
  }

With

  if ($node->default_qty > 0 && variable_get('uc_product_add_to_cart_qty', FALSE)) {
    $form['qty'] = array('#type' => 'select',
      '#title' => t('Quantity'),
      '#default_value' => $node->default_qty,
      '#options' => array(1=>'1',2=>'2',3=>'3',4=>'4',5=>'5',6=>'6',7=>'7',8=>'8',9=>'9',10=>'10',)
    );
  }

Substitue the array with whichever values you would like the drop down to offer. It's that simple :)