You can control how many preorder units are available by setting a limit in your rule. The steps depend on which version of the app you're using.
If You're Using Magical Preorder (the new version):
You can control how many preorder units are available by setting a limit in your rule.
In the app, click Rules.
Open an existing rule or click Create a rule.
Make sure the Active stage is enabled.
Turn on Preorder variant sold out.
Enter your limit in the Preorder limit field.
Click Save.
Once the limit is reached, the product will show as sold out.
If You're Using Preorder Pro + Back in Stock (the legacy version):
In the app, click Home from the left-hand menu.
Find the product you want to manage.
Click the gear icon on the right-hand side of the product.
Toggle Pre-Order Quantity Limit and enter the max number of preorders you want to allow.
Click Save.