When I am listing the recently called numbers, how do I add such a number to the contact list?
I am using the latest Samsung S8, so I will assume yours is most likely exactly the same procedure.
From the Recents list, tap the number you wish to create as a contact. Mine opens a bar just below, 'Call' with a green phone icon, 'Message' with a bubble for messaging, and 'Details' with an info icon.
Tap 'Details' for an action bar to appear at the bottom of the screen with a '+ Create Contact' label. Tap, supply data to the page presented including any Name and Location and Detail information, then tap Save which appears in the header below the notifications area of the screen.