Contacts icon accidentally deleted. How do I get it back?

See the following from Remove built-in Apple apps from the Home screen on your iOS device with iOS 10 - Apple Support

Restore a built-in app that you removed

1. Go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
3. Tap to restore the app.
4. Wait for the app to restore, then open it from your Home screen.