The Customer Voice import manager only allows 500 customers to be imported or uploaded at a time. A workaround is to upload your customer list in Business App instead:
1) Navigate to the Business App, click on the "Customers" tab on the left side panel.
2) Click on "Add", and select "Bulk Add Customers".
3) The Business App customer list importer allows up to 10,000 customers to be uploaded at a time. This list is automatically synced with Customer Voice.
Note: The CSV file must have either an email address or phone number with the country code on each row.