How to add additional "items" on a webform in Drupal 7
The module Webform Add More does similar thing,


  Webform Add More enables selected fieldsets to be initially hidden and
  progressively shown with an Add more button, mimicking CCK multiple
  fields. The approach was proposed by Bartezz, and code originally
  implemented by span and kranklin 1. This module adds a checkbox
  setting to webform fieldset components to select which should have Add
  more functionality, as well as an advanced setting to override the
  button label per Webform.