Adding the iPad to your account, I believe, would require adding it as a line/device, via the device ID, IMEI/MEID and SIM Card. If the device was already on your account, but deactivated, it should/might show up under DEVICES when you log in to MyVerizon.com. How are you trying to activate it where you are being asked for the number?
If the iPad was deactivated, the number associated with the SIM card was lost. The iPad will need a new SIM card and phone number to add 4G LTE service to it again.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
lynda16, your business is greatly appreciated. Ann154 provided some great information. We recommend calling our sales team at 800-256-4646 for additional assistance.