i deleted my mail icon on my iphone 8 and don't know how to recover it

Apps can be removed from the Home Screen without being deleted. Go to the App Library (swipe left past your last Home Screen). From there you can touch and hold an app and slide it to the Home Screen.

If it was delete, see the following from Delete built-in Apple apps from your iPhone, iPad, or Apple Watch - Apple Support

Restore a built-in app that you deleted

1. On your iPhone, iPad, or Apple Watch, go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app.
3. Tap the Redownload button .
4. Wait for the app to restore, then open it from your Home Screen.