NFT artists can mint as many NFTs as they like from one artwork.
Some NFT art will be one of one (a unique piece), whilst some may be a limited edition (e.g. a set of 10, 50 or 100), or open edition (an unlimited number, minted during a limited time period).
There are pros and cons to each approach. One of one NFT art may initially see lower values, but can more easily rise over time as prominence, utility and demand grow. Open edition NFT art may fetch higher initial values when minted, but may not be as successful long-term.
When exploring NFT art on a popular marketplace such as OpenSea, information such as the number of NFTs available, the floor price, description and more will be available to view.