Is the X-mailer number assigned to a specific iPad? On the iPad is there going to be the same X-mailer number assigned to it, or does it change when emails are sent?

Basically, is the X-mailer number assigned to a specific iPad?
The X-Mailer header is used to identify the client application that is sending the message.

If there is a number in this header it should be the version number of the client mail application.  It is not unique to your iPad, all emails sent from the same device model with the same version of the Mail app will have the same number.