To prevent shoppers from adding more than the available preorder inventory to their cart:
Go to the Default Settings tab.
In the “Stock Warning” section, check “Prevent Customers from Ordering More than the Preorder Stock Available.”
Click “Save.”
Important Notes
This feature only works if you're using Inventory Management Option 1 or the Specific Preorder Stock feature.
It only restricts shoppers from adding more than the available preorder inventory on the product page.
It does not prevent customers from manually increasing the quantity on the cart page.