Talmud for Sanhedrin 1:6
סַנְהֶדְרִי גְדוֹלָה הָיְתָה שֶׁל שִׁבְעִים וְאֶחָד, וּקְטַנָּה שֶׁל עֶשְׂרִים וּשְׁלֹשָׁה. וּמִנַּיִן לַגְּדוֹלָה שֶׁהִיא שֶׁל שִׁבְעִים וְאֶחָד, שֶׁנֶּאֱמַר (במדבר יא) אֶסְפָה לִּי שִׁבְעִים אִישׁ מִזִּקְנֵי יִשְׂרָאֵל, וּמֹשֶׁה עַל גַּבֵּיהֶן, הֲרֵי שִׁבְעִים וְאֶחָד. רַבִּי יְהוּדָה אוֹמֵר, שִׁבְעִים. וּמִנַּיִן לַקְּטַנָּה שֶׁהִיא שֶׁל עֶשְׂרִים וּשְׁלֹשָׁה, שֶׁנֶּאֱמַר (שם לה) וְשָׁפְטוּ הָעֵדָה וְגוֹ' וְהִצִּילוּ הָעֵדָה, עֵדָה שׁוֹפֶטֶת וְעֵדָה מַצֶּלֶת, הֲרֵי כָאן עֶשְׂרִים. וּמִנַּיִן לָעֵדָה שֶׁהִיא עֲשָׂרָה, שֶׁנֶּאֱמַר (שם יד) עַד מָתַי לָעֵדָה הָרָעָה הַזֹּאת, יָצְאוּ יְהוֹשֻׁעַ וְכָלֵב. וּמִנַּיִן לְהָבִיא עוֹד שְׁלֹשָׁה, מִמַּשְׁמַע שֶׁנֶּאֱמַר (שמות כג) לֹא תִהְיֶה אַחֲרֵי רַבִּים לְרָעֹת, שׁוֹמֵעַ אֲנִי שֶׁאֶהְיֶה עִמָּהֶם לְטוֹבָה, אִם כֵּן לָמָּה נֶאֱמַר (שם) אַחֲרֵי רַבִּים לְהַטֹּת, לֹא כְהַטָּיָתְךָ לְטוֹבָה הַטָּיָתְךָ לְרָעָה. הַטָּיָתְךָ לְטוֹבָה עַל פִּי אֶחָד, הַטָּיָתְךָ לְרָעָה עַל פִּי שְׁנַיִם, וְאֵין בֵּית דִּין שָׁקוּל, מוֹסִיפִין עֲלֵיהֶם עוֹד אֶחָד, הֲרֵי כָאן עֶשְׂרִים וּשְׁלֹשָׁה. וְכַמָּה יְהֵא בְעִיר וּתְהֵא רְאוּיָה לְסַנְהֶדְרִין, מֵאָה וְעֶשְׂרִים. רַבִּי נְחֶמְיָה אוֹמֵר, מָאתַיִם וּשְׁלשִׁים, כְּנֶגֶד שָׂרֵי עֲשָׂרוֹת:
The Great Sanhedrin was composed of seventy-one judges, and the lesser sanhedrin, of twenty-three. Whence is it derived that the Great Sanhedrin consisted of seventy-one? From (Numbers 11:16): "Gather unto Me seventy men of the elders of Israel," and Moses over them, [viz. (Ibid 17): "And they shall bear with you" — together with you], making seventy-one. R. Yehudah says: Seventy. [He expounds "with you" as "similar to you," and not that he must sit with them in judgment. The halachah is not in accordance with R. Yehudah.] And whence is it derived that the lesser consisted of twenty-three? From (Ibid. 35:24-25): "And the congregation shall judge … and the congregation shall rescue." The congregation judges [i.e., Ten rule guilty], and the congregation rescues [i.e., Ten rule innocent], making twenty. [i.e., We derive from this that there must be twenty. If they are split, ten rule guilty, and ten, innocent.] And whence is it derived that a "congregation" is ten? From (Ibid. 14:27): "How long for this evil congregation" (of the spies), Yehoshua and Calev being excluded (from the twelve). And whence are the three additional derived? From (Exodus 23:2): "Do not be after many for ill," I understand that I should be with them for good. Why, then, need I be told (Ibid.): "After many to incline"? (To be taught:) Not as your inclining for good (acquittal) is your inclining for ill (conviction). Your inclining for good is with (a majority of) one; your inclining for ill requires (a majority of) two (beyond the minimum ten). And since we cannot have a split beth-din, we add another one, making twenty-three. [The verse is understood thus: "Do not be after many for ill," to convict by a majority of one over those ruling for acquittal; but "after many to incline" — with two, even for ill, when there are two more for conviction than for acquittal. Therefore, perforce, twenty-three are required. For there cannot be fewer than ten ruling for acquittal — viz.: "And the congregation shall rescue," so that conviction cannot obtain with fewer than twelve. ("and since we cannot have a split beth-din":) Beth-din cannot be even-numbered; for if (it were, and) they were split (in their decision), we would have half against half, so that "Your inclining for good is with one" could not obtain. Therefore, an additional judge is added, making twenty-three.] And how many must there be in a city so that it qualify for a sanhedrin (of twenty-three)? One hundred and twenty. [The Gemara explains: Twenty-three, the small sanhedrin; three rows of twenty-three each sitting before them, from which to add to the judges if this becomes necessary (see 4:4); ten "idlers" (i.e., ten who are idle from all work and sit constantly in the house of study; two scribes to record the words of those who rule for acquittal and those who rule for conviction; two chazanim, sextons of beth-din, to administer stripes where prescribed and to summon the litigants; two litigants; two witnesses; two who declare them zomemin ("scheming"); two who declare the zomemin, zomemin; two (charity)collectors and a third to distribute the charity (charity being collected by two and being distributed by three); a blood-letter, a scribe; and a teacher of young children — making one hundred and twenty.] R. Nechemiah says: Two hundred and thirty, corresponding to "officers of tens" [i.e., twenty-three tens — so that each judge be an officer of ten, less than that not being regarded as "authority." The halachah is not in accordance with R. Nechemiah.]