You can use the If PowerShell script returns true condition in the Scheduled Task that runs the script for importing users from the CSV file. If the CSV file contains more records than allowed for processing (e.g. 100), executing the import script will require an approval. If the number of records is less than allowed, the import will be performed directly without approval. Finally, the Scheduled Task will look like the following:
Should this approach meet your needs, we will provide you with the script to be used in the condition.
If this is not what you need, please, provide us with a live example of the desired behavior with all the possible details.