A pledge manager is a website separate from Kickstarter that tracks all the details of your pledge after the campaign is over. You use it to update your address and pay for shipping after the campaign.
You can add multiple copies of any tier by just adding multiple of the pledge (2x, 3x, etc) for the number of copies you want. You will also be able to add more copies and individually include add-ons using the pledge manager. Additional shipping charges may apply.