If you pay any attention to the American justice system (or if you've just seen 12 Angry Men), you probably know that the standard number of jurors in most trials is 12. In the 19th century, the United States Supreme Court stated that having exactly 12 jurors hear a case was the defendant's constitutional right. But in 1969, the Supreme Court was faced with the constitutionality of a Florida law that allowed defendants to be convicted after a trial by only 6 jurors in the case of Williams v. Florida. The highest court in the land changed its course and held that 6 jurors were now sufficient to satisfy the constitutional requirements of a trial by jury. But a few years later, the Court said that 5 jurors were not enough to pass constitutional scrutiny.
While the Supreme Court seems to have generally ended the debate about how many jurors are currently required, it does not answer a potentially more important question: how many jurors are ideal? In Williams v. Florida, the Court noted in its opinion that the general belief that the proper number of jurors is 12 was "a historical accident." If this was just an accident, rather than a carefully analyzed number, how can we be sure that we've arrived at the right conclusion?
There have been several studies about the ideal jury size. Generally, studies have found that it's more likely that a larger jury would come to the correct result (especially if you assume each juror has the same odds of voting "correctly"). The problem with larger sizes, though, is that criminal convictions require a unanimous verdict (usually). With that in mind, a jury with 100 members might never convict, and the justice system may grind to a halt. Some studies, in fact, believe that a single juror might be equally capable of deciding cases correctly.
But other studies have indicated that smaller juries represent significant threats to fairness. Juries featuring only 6 members have much lower chance of containing minorities. Coupled with the serious power of attorneys to whittle juries as they deem fit, a smaller resulting jury pool may mean that a devious attorney could so fine-tune a jury as to eliminate any meaningful cross-section of society. 12 Jurors may serve to safeguard minority defendants.
On the other hand, 12 may not be enough. In 2011, the Department of Justice reported that criminal cases have a 93% conviction rate. Hopefully, this number is due to the fact that cases have simply been investigated fully before trials and that prosecutors rarely make mistakes. However, the conviction rate is significantly higher than it was in 1972 (75%). Furthermore, it is almost undisputed that some convictions are, in fact, improper. People are released from prison every year because new evidence has shown that they must be innocent. With more jurors for prosecutors to convince in order to obtain a conviction, we might be able to end some erroneous convictions.
Unfortunately, larger juries aren't free. Tax dollars are required to procure each jury member, and many juries unquestionably incur a serious economic hit by being forced to skip their paying jobs in favor of spending a week hearing a trial. It may bee entirely too cumbersome to require juries of 15, 18, or 24.
With potential costs associated with more jurors, issues with the requirement of unanimity, and general disagreements about how many jurors are actually best, it appears that--for now--12 is still the magic number.