The difference in cost of minting a single NFT and the cost of minting multiple NFTs as a part of one order is not linear. Minting cost can be broken down as follows

Blockchain Operation Cost + Blockchain Transaction Cost + Platform Cost

Minting a single NFT and multple NFTs will have the same Transaction Cost and a different Operation Cost. Platform Cost is fixed for each operation.

The Operation Cost of minting the a single NFT is very similar to the Operation Cost of minting multple NFTs.

TL;DR - If you have more than 5 NFTs - mint them as a batch in a single API operation.