Commerce and Attribute Quantities
Commerce by default supports line item fields that can appear on the Add to Cart form. If you only sell this one type of product on the site and will always need that "additional pages" field, then you could just add the field to the default product line item type. If you have other needs, you'd use a custom line item type either defined in code or created via https://drupal.org/project/commerce_custom_product.

Once you have customers putting in their custom number of pages, it's up to you to adjust the price accordingly. Pricing rules receive the full line item object for price manipulation, including the values of any line item fields present on the Add to Cart form. You can construct a variety of rules that look at the referenced product to determine how much to charge for additional pages and work it out using the Rules actions to create data values.