The set up differs slightly for primary (drops) and marketplace sales:

Pass a quantity attribute to the mintConfig indicating the total number of NFTs and update the totalPrice accordingly.

The price on the mintConfig must match the price on the contract. For example, if you are selling 2 NFTs for 1 ETH each, the totalPrice should be 2 ETH.
For EVM contracts, ensure the attribute name matches the parameter name in your mint function. For example: If your mint function has the signature: mintTo(address _to, uint256 _amount) then the attribute you set in the mintConfig must be _amount instead of quantity.
Make sure that all NFTs belong to the same collectionId and blockchain.