Parameter Description: EmailCC (Email CC Field) sets the value of the CC field.
You can also use dynamic field content for the parameter value. Example: &EmailCC={!Contact.OtherEmailAddress}. You can also include more than one email address, such as: &EmailCC=support@congamerge.com,{!Contact.OtherEmailAddress}.
You can add up to 25 recipients for the Email CC field in each SingleEmailMessage using Salesforce email.