If you are chatting with an unsaved contact and want to save their number, follow these steps:
If you still do not see this contact listed on your Favorites tab after 30 seconds, go to Favorites > Options > Refresh favorites.
WhatsApp uses the phone numbers from your phone's address book. Contacts should be automatically added to your WhatsApp Favorites if you have their numbers entered correctly. Read this article for information on how to format an international phone number.
To learn how to delete a contact, read this article.
WhatsApp Support Team