Can’t find my iPads number to sign in with it I have service with verizon

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features.

An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added a DataSIM and/or Cellular plan to a WiFi+Cellular model of iPad using the built-in eSIM, the number associated with the SIM is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.

If you have installed a DataSIM and/or activated Cellular service on your iPad, you can find the Cellular number used for accounting in iPad settings:

Settings > General > About > Mobile Data Number

An iPad does not have a telephone number - it is not a phone. You can use itlike one if it is associated with an iPhone.