ipad facetime - which number?

Use the parent's e-mail address (Apple ID)

FYI, A standalone iPad even with a cellular model (without an iPhone) cannot use phone number since the iPad is not a phone.

If your parent also has an iPhone using the same Apple ID then you can let others know your parent's phone number.

You should be able to contact a FaceTime user using any of the contact methods enabled in their own FaceTime Settings:

Settings > FaceTime > (You Can Be Reached By FaceTime At]

Your Apple devices and the FaceTime service are intelligent - and work this our for you. If the Contact is defined in your Contacts, you should see a blue FaceTime icon; simply tap - and the call in initiated without you needing to know which addresses/numbers have been configured in their settings.

FaceTime is Apple proprietary requiring an Apple device. The are other similar cross-platform third-party Apps that offer similar features.