The Mishnah (Sanhedrin 1:1) arrives at the number twenty-three based on an exegetical derivation: It must be possible for a "community" to vote for both conviction and exoneration (Numbers35:24–5). The minimum size of a "community" is 10 (Numbers14:27; i.e. the 10 spies). One more is required to achieve a majority (11-10), but a simple majority cannot convict (Exodus23:2), and so an additional judge is required (12-10). Finally, a court should not have an even number of judges to prevent deadlocks; thus 23.
