Changing the supply quantity on Ethereum
You can create fungible items* on Ethereum (items with many copies) by setting the number next to Supply to be greater than one. That will be the maximum number of copies you will be able to mint for the token, a.k.a. the “supply cap”.
The field is disabled by default, but can be enabled by adding ?enable_supply=true to the end of the URL. Just remember to make listings with "Quantity" set to one so that buyers can choose to buy only one copy if they choose.
A single listing of twenty copies can only be purchased if they buyer buys all twenty. This limitation will be eliminated soon.
*This solution only applies to ERC1155 NFTs on Ethereum.