How do I add the order number in the invoice title?
Go to Invoices > Settings > WooCommerce > General. Check the Sequential Order Numbers option, which allows you to give WooCommerce orders sequential order numbers.
Next, go to Invoices > Settings > Documents > Invoice and delete the contents of the Prefix and Suffix fields. In the next number field, add the next order number.
With these settings, the next order number will be added in the invoice title.