"How do I find my Kickstarter Backer Number?"
To help us find your pledge, the best option is to provide your unique Kickstarter backer number. Here's how you can find it.
These instructions are copied from Kickstarter's FAQ on this topic:
Where can a backer find their backer number?
Backers will be able to locate their backer number in their pledge confirmation email (the email subject will be “You just backed [project name]”).
Folks with a Kickstarter account can also find their backer number by going to their Backed Projects and clicking on the blue “+” button for the corresponding project. This will expand the details of their pledge and the backer number can be found at the bottom.
Alternatively, after a project has ended successfully, backers will be able to find their backer number by clicking “View pledge” in the blue banner at the top of the project page.