Twelve is a traditional number that has just stuck over the past several centuries. But it does not have to be twelve on a jury. The number of jurors must be a specified size (usually twelve, though smaller cases may require only six), and since there is always the possibility of jurors not completing the trial for health or other reasons, often some alternate jurors are nominated, who will also follow the trial (but do not take part in deciding the verdict), as a precaution in case a new juror is needed part way through the trial (most often used when the trial will be lengthy or high-profile). Some additionial information: Here in the US, the jury was an essential safeguard of liberty long before the American Revolution. British courts guaranteed the independence of criminal trial juries in 1670, in a case concerning four jurors who had acquitted William Penn for illegally preaching about his Quaker beliefs. Those jurors were imprisoned for their "not guilty" verdict because they had ignored the trial judge's instructions to vote for Penn's conviction. An English appellate court released the jurors from prison, establishing the principle that juries cannot be punished for bringing in the "wrong" verdict. The freedom of American jurors to vote according to conscience can be traced to that landmark precedent. In the United States every person accused of a felony has a constitutional right to a trial by jury, which arises from the 6th amendment (made applicable to the states through the Fourteenth Amendment) that states in part: "In all criminal prosecutions, the accused shall enjoy the right to a speedy and public trial, by an impartial jury of the state and district wherein the crime shall have been committed..." Most states' constitutions also grant the right of trial by jury in lesser criminal matters, though most have abrogated that right in offenses punishable by fine only. http://www.cato.org/dailys/12-09-98.html

The significance of twelve comes from Biblical times, and Jesus' 12 disciples.

I believe it comes from Bible. In Exodus, The Lord sent Word to Israel via Moses to Judge and manages the Israel people. Exodus (24:3) And Moses came and told the people all the words of the LORD, and all the judgments: and all the people answered with one voice, and said, All the words which the LORD hath said will we do. Exodus (24:4) And Moses wrote all the words of the LORD, and rose up early in the morning, and builded an altar under the hill, and twelve pillars, according to the twelve tribes of Israel. The legal System in west countries are based on the Bible.

1) "12 disciples: As well as the 12 disciples and 12 days of Christmas, the number has a significance for other religions. In Judaism, there were Twelve Tribes of Israel and a girl reaches maturity (bat mitzvah) at 12, a year earlier than for boys. In Islam, the largest Shi'ite sect are the Twelvers, who accept 12 of the descendants of Mohammed's right-hand man Ali as divinely ordained leaders, or Imams. 12 good men and true: The idea of settling disputes using a jury of 12 impartial witnesses is usually dated back to King Aethelred's Wantage code of 933, although some scholars point out that the English jury system that emerged in the 12th century was almost identical to the Islamic Lafif, which the Normans may have brought from Sicily." Source and further information: http://www.telegraph.co.uk/portal/main.jhtml?xml=/portal/2008/08/30/ftqi130.xml 2) "A thought provoking article on BBC asks the question: is the British Law based on the Muslim law, and did the Knight Templars have something to do with it. Of course, a lot of these sort of questions are just that: questions, but they give one pause to think." "For some scholars, a historical connection to Islam is a "missing link" that explains why English common law is so different from classical Roman legal systems that hold sway across much of the rest of Europe." "From the end of the 9th to the middle of the 11th Century, Sicily had Muslim rulers. Many Sicilians were Muslims and followed the Maliki school of legal thought in Sunni Islam. Maliki law has certain provisions which resemble English legal principles, such as jury trial and land possession. Sicily represented a gateway into western Europe for Islamic ideas but it's unclear how these ideas are meant to have travelled to England. Norman barons first invaded Sicily in 1061 - five years before William the Conqueror invaded England. The Norman leaders in Sicily went on to develop close cultural affinities with the Arabs, and these Normans were blood relations of Henry II, the English king credited with founding the common law. But does that mean medieval England somehow adopted Muslim legal ideas? Merton College was founded on principles similar to Islamic law There is no definitive proof, because very few documents survive from the period. All we have is the stories of people like Thomas Brown - an Englishman who was part of the Sicilian government, where he was known in Arabic as "Qaid Brun". He later returned to England and worked for the king during the period when common law came into being. There is proof he brought Islamic knowledge back to England, especially in mathematics. But no particular proof he brought legal concepts. There are clear parallels between Islamic legal history and English law, but unless new historical evidence comes to light, the link remains unproven." Source and further information: http://www.chowk.com/ilogs/68996/46049 3) "The Islamic Lafif was a body of twelve members drawn from the neighbourhood and sworn to tell the truth, who were bound to give a unanimous verdict, about matters "which they had personally seen or heard, binding on the judge, to settle the truth concerning facts in a case, between ordinary people, and obtained as of right by the plaintiff." The only characteristic of the English jury which the Islamic Lafif lacked was the "judicial writ directing the jury to be summoned and directing the bailiff to hear its recognition." According to Professor John Makdisi, "no other institution in any legal institution studied to date shares all of these characteristics with the English jury." It is thus likely that the concept of the Lafif may have been introduced to England by the Normans, who conquered both England and the Emirate of Sicily, and then evolved into the modern English jury.[6] Several other fundamental common law institutions may have been adapted from similar legal institutions in Islamic law and jurisprudence, and introduced to England by the Normans after the Norman conquest of England and the Emirate of Sicily, and by Crusaders during the Crusades. In particular, the "royal English contract protected by the action of debt is identified with the Islamic Aqd, the English assize of novel disseisin is identified with the Islamic Istihqaq, and the English jury is identified with the Islamic Lafif." Other English legal institutions such as "the scholastic method, the license to teach," the "law schools known as Inns of Court in England and Madrasas in Islam" and the "European commenda" (Islamic Qirad) may have also originated from Islamic law. The methodology of legal precedence and reasoning by analogy (Qiyas) are also similar in both the Islamic and common law systems. These influences have led some scholars to suggest that Islamic law may have laid the foundations for "the common law as an integrated whole"." Source and further information: http://en.wikipedia.org/wiki/Fiqh Further information: http://en.wikipedia.org/wiki/Common_law#Possible_influence_of_medieval_Islamic_law 4) "The twelve senior thegns of the hundred play a part, the nature of which is rather doubtful, in the development of the English system of justice. By a law of Aethelred they "seem to have acted as the judicial committee of the court for the purposes of accusation" (W.S. Holdsworth, History of English Law, vol. i. 1903), and thus they have some connexion with the grand jury of modern times." Source and further information: http://en.wikipedia.org/wiki/Thegn