ERC-721 Non-Fungible Token Standard, optional supply extension
See https://eips.ethereum.org/EIPS/eip-721 Note: the ERC-165 identifier for this interface is 0x780e9d63.
function totalSupply() external view returns (uint256)
Count NFTs tracked by this contract
|_0||uint256||A count of valid NFTs tracked by this contract, where each one of them has an assigned and queryable owner not equal to the zero address|