To be judged by 12 is to go to court and to be judged by a 12 man jury, which all court trials have.
To be carried by 6 is to be dead. It is common for 6
people to carry a coffin (3 on each side) to and from the hearse.
In total the phrase "I'd rather be judged by 12, than carried by 6" means that the
person would rather kill someone and go to
prison than to, themself, be killed and have to be buried.