If you're refering to your uphone number thats set up to be used with imessage on your ipad, then that is associated with your apple id and has to be changed there first.
My Apple ID
Then on your ipad, sign out of messages. Go to settings/messages/tap on send and receive/tap on apple id/sign out
Then sign out of messages on phone and then back in.
Then sign into messages on ipad again, number should appear