Scarcity of NFT
The creator of an NFT is in charge of determining the asset's scarcity.
Consider purchasing a ticket to a sporting event. The designer of an NFT can pick how many replicas there are, much as an event organizer can choose how many tickets to sell. 5000 General Admission tickets, for example, are sometimes exact reproductions. A ticket with an assigned seat, for example, maybe issued in multiples that are extremely similar yet somewhat different. In another scenario, the designer might seek to construct a one-of-a-kind NFT as a rare collectible.
Each NFT would still have a unique identity (like a bar code on a typical "ticket") and only one owner in these scenarios.
The NFT's intended scarcity is important, and it is up to the designer to decide. A creator may plan to make each NFT fully unique to promote scarcity, or they may have a good cause to make thousands of copies. Keep in mind that all of this information is available to the general public.