Whatever appears in the body of the letter e.g. "...and I will send a copy to (list of the 12 people)", you would still need to give the 12 names in the cc section.
My only suggestion would be, does your boss have to give the names in the body of the letter - can he/she say, I will forward this information to the other member of the team/committee and.."
It is then apparent from the names in the cc section who these team members are.
If on the other hand, the letter is informing the person, just who the members are comprising the team, and the cc letters are letting the team know she has written to this other person, that's it I'm afraid - just type the names once, and Cut and Paste into the cc section!
Otherwise, it is two separate letters. One to this 'person', without a cc, and one to the next most senior team member, with cc to the rest.