I can currently mint a single NFT using a node.js script.
How can I mint, say 10 NFTs, in a single operation (and avoid costs in Tez that multiple operations would cause)?
Minting operation is not part of FA2 standard.
Answer is to have a contract that allow you to mint several NFT in only one operation. Note that it wouldn't reduce the gas, but you can pay baker fees only once.
If your contract mint 1 NFT per operation, you cannot. And if you want to mint 10 NFT, in 10 operations in the same block, you would use batch operations
FA2 does NOT specify an interface for mint and burn operations; however, if an FA2 token contract implements mint and burn operations, it SHOULD, when possible, enforce the same logic (core transfer behavior and transfer permission logic) applied to the token transfer operation. Mint and burn can be considered special cases of the transfer. Although, it is possible that mint and burn have more or less restrictive rules than the regular transfer. For instance, mint and burn operations may be invoked by a special privileged administrative address only. In this case, regular operator restrictions may not be applicable.