How to add form fields dynamically using webform
You can use Webform Add More module to dynamically add more fields in webform.
The project page says:


  Webform Add More enables selected fieldsets to be initially hidden and
  progressively shown with an Add more button, mimicking CCK multiple
  fields. 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