Nowhere in the U.S. Constitution does it say that juries in criminal cases must include 12 people, or that their decisions must be unanimous. So where did we get 12, the number of people that must be on a jury. And why do some states use juries of different sizes.
Jury Trials were established in 725 A.D. by The Welsh king Morgan of Glamorgan. He decided upon this number, linking the judge and jury to Jesus and his 12 apostles. The evolution of the jury in the United States has roots in English law. Not surprisingly, after the Revolution the newly independent Americans were very aware of the importance of a jury for trials. This was particularly true after their experience with political oppression. Thomas Jefferson wrote, “Were I called upon to decide whether the people had best be omitted in the legislative or judiciary department, I would say, it is better to leave them out of the legislative. The execution of the laws is more important than the making them.”
The Supreme Court ruled in 1978 that a 5 person jury is not allowed after Georgia attempted to assign five-person juries to certain criminal trials. The Fifth, Sixth and Seventh Amendments to the U.S. Constitution guaranteed the right to a jury for all criminal cases, and in all civil suits exceeding twenty dollars. Read more here on the evolution of juries.