Is it possible to delete a template from numbers?

I'll assume you're using Lion or maybe Mountain Lion. Numbers stores those you created & saved as templates in (your account) > Library > Application Support > iWork > Numbers > Templates > My Templates. The user's Library is hidden in Lion but it is easy to open. In Finder, hold down the Option key while clicking on the Go menu & your user Library will appear about halfway down the list.

I think you're looking in the wrong library, it's a common problem. Open a Finder window in column view & click on the house icon for your account. Now follow the path I gave above.