Is it possible to use random submission number for webform?
You are probably looking for something like Webform Submission UUID module:


  This module provides a UUID integration for Webform Submissions


UUIDs are hashes that looks random and are not based on submission order, so they would meet your need. Note that this module is in dev version only, so you can expect errors. On the other hand it's GPL, so you can use it as an example how to interact with submission IDs.