How to limit available amount for preorders?
To limit preorders overall you can set the total available quantity in the product's Inventory Management and uncheck Continue selling when out of stock.
For example, if you want to have 10 available items only:
Then in the app, you select Mode 1. Make sure to set Mode 1 per product or collection. If you set Mode 1 in Global settings make sure to disable 'Enable preorders for all products'. Otherwise, the Pre-order button will show on all products.
This will show the item as preorder until the quantity gets to 0. Afterward, it will show as Sold Out and will be unavailable for purchase.