I completely agree with Jen's answer and I am writing here to discuss some deeper historical dimensions to the question.
Historically, notion of using a 23 member grand jury dates to the reign of King Edward III whose reign started in the year 1368 in England.
It is not derived from the 23 members of the Lesser Sanhedrins in ancient Israel. The grand jury was changed from having 12 members to having 23 members at least seventy-eight years after King Edward I expelled all Jews from England in the year 1290, at a time when the expulsion of Jews from England was still in effect.
The Grand Jury can also be traced to the time of the Norman conquest
of England in 1066. There is evidence that the courts of that time
summoned a body of sworn neighbors to present crimes that had come to
their knowledge. Since the members of that accusing jury were selected
from small jurisdictions, it was natural that they could present
accusations based on their personal knowledge.
Historians agree that the Assize [court session or assembly] of
Clarendon in 1166 provided the ground work for our present Grand Jury
system. During the reign of Henry II (1154-1189), to regain for the
crown the powers usurped by Thomas Becket, Chancellor of England, 12
"good and lawful men" in each village were assembled to reveal the
names of those suspected of crimes. It was during this same period
that juries were divided into two types, civil and criminal, with the
development of each influencing the other.
The oath taken by these jurors provided that they would carry out
their duties faithfully, that they would aggrieve no one through
enmity nor deference to anyone through love, and that they would
conceal those things that they had heard.
By the year 1290, these accusing juries were given the authority to
inquire into the maintenance of bridges and highways, defects of
jails, and whether the Sheriff had kept in jail anyone who should have
been brought before the justices. "Le Grand Inquest" evolved during
the reign of Edward III (1368), when the "accusatory jury" was
increased in number from 12 to 23, with a majority vote necessary to
indict anyone accused of crime.
A grand jury is an ex parte proceeding run by a prosecutor in secret with only the most minimal judicial involvement. Counsel for the suspects can represent the suspects as witnesses if they are called to testify before the grand jury, but cannot make arguments on applying the law to the facts or legal arguments to a grand jury.
To indict, twelve members of a grand jury must vote to indict (no matter how many members the grand jury has), and a majority of the grand jury must support the indictment (which is why 23 grand jurors is the upper limit). So, for example, in a federal grand jury with 16 members, three quarters of them must vote in favor of doing so to indict a suspect.
The increase in the size of the grand jury from twelve to twenty-three made it easier to indict criminal defendants by relaxing the unanimity requirement for an indictment that had existed when there were only twelve jurors on a grand jury.
The grand jury is supposed to vote to indict if it finds that there is probable cause to find that a defendant committed a crime under the law explained to it by the prosecutor and the evidence that is presented to it by the prosecutor. One grand jury's refusal to indict a suspect does not preclude a prosecutor from presenting the same charges against the same suspect to a different grand jury later on.
Federal grand juries almost always vote to indict. At the state level, indictment rates vary widely from one county to another, and from state to state.
The grand jury requirement exists in the U.S. under state law mostly in states in the Eastern U.S., while most Western states allow grand juries to be convened but only do so in exceptional cases (usually for political reasons or as a secret means for prosecutors to gather evidence).
In the early decades of the United States grand juries played a major
role in public matters. During that period counties followed the
traditional practice of requiring all decisions be made by at least
twelve of the grand jurors, (e.g., for a twenty-three-person grand
jury, twelve people would constitute a bare majority).
Any citizen could bring a matter before a grand jury directly, from a
public work that needed repair, to the delinquent conduct of a public
official, to a complaint of a crime, and grand juries could conduct
their own investigations. In that era most criminal prosecutions were
conducted by private parties, either a law enforcement officer, a
lawyer hired by a crime victim or their family, or even by laymen. A
layman could bring a bill of indictment to the grand jury; if the
grand jury found there was sufficient evidence for a trial, that the
act was a crime under law, and that the court had jurisdiction, it
would return the indictment to the complainant.
The grand jury would then appoint the complaining party to exercise
the authority of an attorney general, that is, one having a general
power of attorney to represent the state in the case. The grand jury
served to screen out incompetent or malicious prosecutions. The advent
of official public prosecutors in the later decades of the 19th
century largely displaced private prosecutions.
Grand juries were popular in the Eastern states because in the Revolutionary War period and the period leading up to the Revolutionary War in the United States, grand juries were used as a tool of resistance to what was perceived as oppressive British rule by citizens of the American colonies.
But, that fervor had faded and the difficulty of finding grand jurors on the frontier discouraged the use of this institution of people migrated to the west to form new states, and as the prosecution of crimes came to be the sole or predominant province of professional prosecutors employed by the state.
In states that don't require grand jury indictments in all felony cases, an adversarial preliminary hearing before a judge to screen for probable cause is used in in lieu of a grand jury in some felony cases.
The only country other than the United States which still uses grand juries is Liberia, which was founded by freed slaves from the U.S.