How do we know that a smaller Sanhedrin, consisting of twenty-three members, is sufficient? As it is stated: "And the congregation shall judge, and they shall deliver the slayer from the hand of the avenger of blood." The term "congregation" indicates that the judgment of the congregation is effective, and the term "congregation" implies a minimum of ten individuals. How do we know to include an additional three? It is derived from the verse: "You shall not follow the majority for evil." I understand that one should be with them for the good, meaning even if there is only one person advocating for the defendant, it is sufficient. However, the verse states: "To pervert [the judgment]"; this implies that even for evil, meaning if there are two people advocating for conviction, the judgment is still effective. And the Sanhedrin, which is not equivalent to them, may not add another member, as the minimum number required is already twenty-three.
Why the Sanhedrin Needed Exactly Twenty-Three Judges
Yalkut Shimoni on Torah 788:4
מִנַּיִן לְסַנְהֶדְרֵי קְטַנָּה שֶׁהִיא עֶשְׂרִים וּשְׁלֹשָׁה. שֶׁנֶּאֱמַר "וְשָׁפְטוּ הָעֵדָה וְהִצִּילוּ הָעֵדָה", עֵדָה שׁוֹפֶטֶת וְעֵדָה מַצֶּלֶת הֲרֵי עֶשְׂרִים, וּמִנַּיִן לְעֵדָה שֶׁהִיא עֲשָׂרָה, שֶׁנֶּאֱמַר (במדבר יד, כז) "עַד מָתַי לָעֵדָה הָרָעָה", יָצְאוּ יְהוֹשֻׁעַ וְכָלֵב, וּמִנַּיִן לְהָבִיא עוֹד שְׁלֹשָׁה, מִמַּשְׁמָע שֶׁנֶּאֱמַר "לֹא תִּהְיֶה אַחֲרֵי רַבִּים לְרָעֹת" שׁוֹמֵעַ אֲנִי שֶׁיִּהְיֶה עִמָּהֶן לְטוֹבָה אִם כֵּן לָמָּה נֶאֱמַר (שם) "אַחֲרֵי רַבִּים לְהַטֹּת", לֹא כְּהַטָּיָתְךָ לְטוֹבָה הַטָּיָתְךָ לְרָעָה, לְטוֹבָה עַל פִּי אֶחָד לְרָעָה עַל פִּי שְׁנַיִם, וְאֵין בֵּית דִּין שָׁקוּל, מוֹסִיפִין עֲלֵיהֶם עוֹד אֶחָד, הֲרֵי עֶשְׂרִים וּשְׁלֹשָׁה.