You can send cc to 25 Marketo Lead or company of type email. You can create a custom "email" type field and include it as a cc option. But, I am sure creating multiple custom field for using as an cc option will not be a good option here.
You can use the CC feature with a static address.
You can't set Reply-To to multiple individual addresses. (It actually is supported by the SMTP standard, but not by Marketo nor by mail clients, so it's a non-starter.)