At least in the US, twelve jurors for criminal trials isn’t a requirement. Few courts however, have more or less than this many. (In 1978, the Supreme Court actually ruled peculiarly a five-person jury is not allowed.)
In general around the world, a jury of twelve for criminal trials has been tradition: for nearly 1300 years.
The year is 725 A.D., in the petty kingdom of Glywsing in southeast Wales, led by King Morgan ab Athrwys.
Establishing the first jury trials ever, he thought it fit to compare the judge and jury to Jesus and His Apostles.
Making his declaration, King Morgan said:
“For as Christ and his 12 apostles were finally to judge the world, so human tribunals should be composed of the king and 12 wise men.”
By the 13th century in England, if an accused person could not produce 12 neighbors, the original jury of one’s peers, to swear to their innocence, they were presumed guilty.
Such a system was problematic for people simply passing through town or lacking social skills, but was surely superior to a duel or the “ordeal,” carrying a red-hot iron and if healed on the third day, judged innocent.
So, God forbid you ever stand accused in a criminal trial, thank Jesus and the Twelve Apostles for having a jury for fair judgement.