How do you retrieve deleted contacts?

That would depend on how you are managing your contacts. If iCloud, see this, iCloud: Restore contacts, calendars, and more in iCloud Settings

If an email account, check with that provider. If you were just storing contacts on your iPhone, then you would need to restore your phone to a backup that was made when the contacts were on the phone.

If you sync contacts to iCloud (Settings/<your name>/icloud - Contacts: ON) you can log in to https://icloud.com, click on your name, then iCloud Settings, and near the bottom of the screen will be an option to restore contacts.

If you do not sync contacts to iCloud you will have to restore a backup of your phone that contains the contacts. This will overwrite everything on your phone, not just contacts.