"How do I find my Kickstarter Backer Number?"
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.