As Cigars said, they are serial numbered cards. Some cards only have a limited amount produced, and each individual card gets a number out of however many created. As such, you may sometimes find the card company getting cute and producing the amount of the card as the year it is released. For instance Topps baseball this year has a gold SN parallel with all of them being XX/2017.
If a card is serial numbered, it should be listed in the database under Notes 1 as SNXX, with the XX being the total number of the cards produced, not the actual number of the card.
So for example, your Joe Smith card should be listed as SN2016, showing that all those cards will be one of those 2,016 cards.