accidentally deleted the mail app on my iphone 12. How do I retrieve it?

ahh.. then you did not delete the app, you probably just 'removed' it

on your Home Screen, swipe left until you get to the App Library. swipe downward and you'll get an alphabetical list of all your apps. scroll down and find Mail. tap/hold/drag and it'll pop back out to your Home Screen

Organize the Home Screen and App Library on your iPhone - Apple Support

Read this

Delete built-in Apple apps on your iOS 12, iOS 13, or iPadOS device or Apple Watch - Apple Support

Here is a link:

https://apps.apple.com/us/app/mail/id1108187098

Or you can just search "Apple Mail app" in App Store on your iPhone.

did you read thru the link rather than just the title of it? the content still applies to iOS 14. Go to the App Store, search for Mail, download it

no problem! glad I could help