https://www.quora.com/How-do-I-find-my-iPhone-number
On an iPhone, go to settings > phone and your phone number is displayed there.

https://www.youtube.com/watch?v=PsodDlH-USA&vl=en
In this video I'll show you how to find your phone number on your iPhone. UPDATED VIDEO: How to Find Your iPhone Phone Number: ...

https://www.youtube.com/watch?v=lu9Ug005a_U
Here's how to find your phone number on your iPhone even if you have multiple eSIMs on your phone or changed the number recently.