Test - 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).

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

How does pre-order quantity limit work?

Inventory is the quantity of a particular product that is available for sale.
Pre-order allows creating a negative inventory value for each sale.
Meaning if you have 10 pre-orders you would end up with a -10 inventory value.
Pre-order button is shown when product inventory in Shopify reaches 0 / is at or below 0.