In accordance with card payments security standards an invoice should never include a full card primary account number (BT-87). At the moment PCI Security Standards Council has defined that the first 6 digits and last 4 digits are the maximum number of digits to be shown.
Explanation
For security reasons (PCI DSS), the full credit card number (BT-87) must not appear on an invoice. Typically only the last 4 to 6 digits are shown.