How can I limit the quantity of products available for pre-order?

There is no limit to the pre-order quantity by default. It means that a customer can pre-order an unlimited quantity of products.
You can set up a pre-order quantity limit, which can be pre-ordered for a specific variant of the product (in the case of applying the settings to the whole product(s) - for each variant).
If the pre-order quantity limit is 10, it is reached when the inventory value is -10.

Set up pre-order quantity limit for one variant or product

1. Go to Products
2. Click  in the variant/product line
   
3. Set up pre-order quantity limit
   
   
4. Click Save

Set up pre-order quantity limit for several variants or products

1. Go to Products
2. Select several variants or products using checkboxes
   
3. Click Apply settings
   
   
4. Set up pre-order quantity limit
   
5. Click Save

Set up pre-order quantity limit by default

1. Go to Settings > Default settings 
   
2. Click Edit
   
   
3. Set up pre-order quantity limit
   
4. Click Save