change the phone number on an iPad

Your iPad doesn’t natively provide any Cellular services - and doesn’t have a phone number. Your iPad may use its Continuity features to send/receive SMS/MMS messaging, or use Cellular WiFi-Calling services provided by your iPhone if you have one.

If you have a Cellular model of iPad, with a Cellular Data plan, the “number” is simply used for billing purposes. As such, it can only be changed by changing your Cellular Service Plan or Cellular provider.