How can I put a FaceTime number in my iPad

For contacting other people:

Seems FaceTime makes use of the entries in the "Contacts" app / just type a name

or : Apple support's article: Use FaceTime with your iPhone or iPad - Apple Support

says:

"In the FaceTime app, tap New FaceTime

and type the person's phone number or email address.

Tap the number or address,

then tap either the audio button (phone) or video button (camera)  FaceTime".

else:

For adding your own phone number:

How to: Add or remove your phone number in Messages or FaceTime

says:

"For FaceTime, go to Settings > FaceTime. If asked, sign in with the Apple ID that you're using with Messages and FaceTime on your iPhone.

To turn on your phone number, select it and your Apple ID in the “You can be reached” section.

If you don't see your phone number, get help with your settings or contact Apple Support. If you see a number that isn't yours, change your Apple ID password."

(If needed: "To turn off your phone number, uncheck it").

All the best :-)