Limiting the quantity of an item that is available for pre-order

Something like Timesact is doing as Stock Management Options:

https://www.youtube.com/watch?v=kexN1w1F6n8

There are three options for how your Pre-Orders will activate depending on your Shopify Inventory availability:

Pre-Order Only When Out of Stock: ‘Pre-Order’ replaces ‘Add to Cart’ when variant stock in Shopify reaches 0. When the stock goes above 0 again, ‘Pre-Order’ is replaced by ‘Add to Cart’.

Pre-order regardless of levels: ‘Add to Cart/Sold Out’ is immediately replaced by ‘Pre-Order’. You can offer unlimited pre-orders or set a quantity limit. When the limit is reached ‘Pre-Order’ reverts to ‘Add to Cart/Sold Out’.

Pre-order only when Shopify inventory is available: ‘Pre-Order’ replaces ‘Add to Cart’ when variant stock in Shopify is above 0. When stock reaches 0, ‘Pre-Order’ is replaced by ‘Sold out’ (customer won’t be able to order anymore) until inventory goes above 0.