The following config will reject a message when more than 10 recipients are in the TO: and/or CC: fields.
/etc/postfix/main.cf:
# restrict based on message header content header_checks = pcre:/etc/postfix/header_checks
/etc/postfix/header_checks:
/^To:([^@]*@){10,}/ REJECT Sorry, your message has too many recepients.
/^Cc:([^@]*@){10,}/ REJECT Sorry, your message has too many recepients.Solution based on a post at linuxquestions.org.