When you use OpenSea’s Lazy Minting tool, you can create digital items with multiple copies (semi-fungible NFTs)* by setting the "supply cap" to anything greater than one.
The supply cap specifies the maximum number of copies you can create for the token and can be set using the Supply field on the "Create New Item" page.
When selling the item, remember to set the Quantity in your listings to one so that buyers are only purchasing one copy at a time.
A single listing of 20 copies can only be purchased if the buyer buys all 20. This limitation will be eliminated soon.
*This solution only applies to ERC-1155 NFTs on Ethereum.