How to use the Webform Serial in a Webform Rule?
Have a look at comment #2 in issue #2572067, which states this:

The correct token for that is [submission:serial], but some module is going to have to make that token available to the rules module. Webform itself doesn't do that. I haven't tried it, but the Webform Rules modules would be the place to start.
The serial number is a relatively new feature, but I would have though it would work without an update to the Webform Rules module. If not, you might file an issue in that module's queue.