This is the number/ID that you gave to your application when you initially created it in iTunes Connect. In order to locate this number again follow these steps:
- Open and log in to iTunes Connect.
- Click Manage your Applications.
- Find and select your application (you may have to click the See All button to find your application).
The string that appears just under the Identifiers area marked SKU is your SKU number. You can just select this string, and copy/paste it to where you need to.