A common number is fifteen.

Some states have no grand jury.

In some others the grand jury is summoned only when requested by the court.
The United States constitution and most of the State constitutions declare that no person shall be held to answer for a criminal offense, except a minor one, "unless on the presentment or indictment of a grand jury." This is to save people from the vexation and expense of arrest and trial unless there is reasonable presumption of their guilt.

On the other hand, a grand jury should aid in bringing to justice persons who indulge in practices subversive of public peace, but which individuals are disinclined to prosecute, such as gambling.

Incidentally the grand jury examines into the condition of the county jail and poor-house.
The mode of selecting grand jurors is in general the same in all the states.