How to add phone number in Send

Some of my texts are sending to some of my contacts and some aren't. My aunty who has been using an iPhone 4 for a while compared our settings and she said that the reason that some of my texts aren't sending was because I didn't have my phone number in the Send & Recieve settings?

I only have an email in my Send & Recieve settings and I was wondering how I add my phone number to it? Please help 🙂 Thank you!