Hello. This is Klaytn API Service team.
Please refer to the following information about the balance of NFT.
NFT stands for Non-Fungible Token, which is not an interchangeable token. Therefore it has unique value and it cannot be interchanged with other tokens. You can always think of NFT as one when it comes to quantity. As its quantity doesn’t have more than one, there's no API to check its quantity.
Klaytn API Service team.