The Sanhedrin were the recognized headship of the Jewish people, in ancient Israel, and they are mentioned in the New Testament, the writings of Josephus, and the tractates of the Mishnah. The Sanhedrin is thought to have originated in the 3rd century . B.C. It was composed of 70 members, mostly priests and Sadducean nobles, some Pharisees, scribes, and elders (leaders of each tribe), presided over by the high priest.
History of the Sanhedrin. The Sanhedrin was an organized Jewish Council of 70 men, these men formed the Supreme Court of ancient Israel. According to the ancient rabbis the Sanhedrin originated with the Council of the 70 elders appointed to assist Moses (Numbers 11:16, M. Sanhedrin 1:6). The rabbis taught that after the exile Ezra re-organized the Council. Historical evidence can only trace the Sanhedrin to about the middle of the third century BC. Josephus mentions the Sanhedrin in relation to Antiochus the Great about 200 BC. In the New Testament times the Sanhedrin was controlled by the wealthy Sadducees and the high priest was president, as indicated in Mark 14:53, and Acts 24:1.
The Meeting Place of the Sanhedrin. There has been little archaeological evidence regarding the Sanhedrin and where they assembled in Jerusalem. According to the Talmud the assembled on the southern side of the Temple compound, in the "Hall of Hewn Stone" (M. Mid. 5:4). When Jesus was on trial the Sanhedrin assembled at the palace of the high priest in the courtyard (Mark 14:53-55), but this does not seem to be the usual circumstances for a meeting.
Wikipedia. Greater and Lesser Sanhedrin. The Talmud (tractate Sanhedrin) identifies two classes of rabbinical courts called Sanhedrin, a Great Sanhedrin (בית דין הגדול) and a Lesser Sanhedrin (בית דין הקטן). Each city could have its own lesser Sanhedrin of 23 judges, but there could be only one Great Sanhedrin of 71, which among other roles acted as the Supreme Court, taking appeals from cases decided by lesser courts. The numbers of judges were predicated on eliminating the possibility of a tie and the last to cast their vote was the head of the court.