Add new ipad with different number to imessager

A cellular+WiFi iPad does not have a useable telephone number. The number associated with your iPad cellular data plan is used by the carrier for internal billing association only.

An iPad cannot connect to any cellular telephone voice network. By itself, it is a data (i.e. internet) only device and thus can only use VOIP telephone services or internet messaging services (iMessage, Skype, etc).

If you have an iPhone you can pair the iPad to the iPhone to share the iPhone’s cellular telephone service including SMS and MMS -> Add or remove your phone number in Messages or FaceTime - Apple Support

An iPad does not have a usable phone number. It is tied to your iPhone if you plan to make calls or receive messages.