To add late fee days in WHMCS, you'll need to follow these steps:

1. Log into WHMCS: Access your WHMCS admin panel using your username and password.

2. Navigate to Configuration: Click on the "Configuration" tab in the top navigation menu.

3. Automation Settings: Under the "System Settings" section, click on "Automation Settings."

4. Late Fee Settings: In the "Automation Settings" section, find the option called "Automatically apply late fees to overdue invoices." Enable this option.

5. Late Fee Days: Below the option you just enabled, you'll see a field labeled "Number of Days After Due Date to Apply Late Fee." Enter the number of days after the due date when you want the late fee to be applied.

6. Late Fee Type (Optional): You may also see an option for "Type of Late Fee". This allows you to choose between a fixed amount or a percentage of the overdue invoice.

7. Late Fee Amount (Optional): If you've chosen a fixed amount, enter the specific late fee amount.

8. Grace Period (Optional): If you want to give clients a grace period before applying late fees, you can specify the number of days in the "Late Fee Grace Period" field.

9. Save Changes: After configuring the late fee settings, click the "Save Changes" button to save your modifications.

10. Notify Clients: You can also configure WHMCS to automatically notify clients when late fees are applied. In the "Automation Settings," find the option "Send Late Fee Notices" and enable it. Configure the notification settings according to your preferences.

Please note that applying late fees is a financial practice and it's important to communicate this policy clearly to your clients. Make sure to include information about late fees in your terms of service or payment policies.

Keep in mind that it's always a good idea to inform your clients in advance about any late fee policy changes to avoid any misunderstandings.