When attempting to run an import-type migration using Insane Mode, either of the following messages is displayed:
The custom permission associated with the item could not be copied since it would exceed the list limit.
The maximum number of permission scope for this list has been reached.
This error is associated with Access Control List (ACL) limits in SharePoint, and SharePoint has a limit of 50,000 custom permissions within a list.
You can apply custom permissions to an item, document, or folder with broken inheritance.
Notes: Since the ShareGate migration tool runs the import asynchronously, doing a pre-check followed by a migration might not give the error on the same items. The app can only identify this error if you run a migration with a 50,000 permissions scope. For example, the migration tool will not display the error if you add 10,000 custom permissions to a list with 45,000 custom permissions.
You will not be able to migrate all your items without changing how you manage the permissions for that list or library.
A quick solution is to remove all the custom permissions at the destination during the migration. To remove these permissions, you can uncheck Permissions in the Configuration section of your copy option.
You can use folders or split your content into multiple lists or libraries to manage your permissions on numerous items.