Related for Megillah 4:2
בְּרָאשֵׁי חֳדָשִׁים וּבְחֻלּוֹ שֶׁל מוֹעֵד, קוֹרִין אַרְבָּעָה, אֵין פּוֹחֲתִין מֵהֶן וְאֵין מוֹסִיפִין עֲלֵיהֶן, וְאֵין מַפְטִירִין בַּנָּבִיא. הַפּוֹתֵחַ וְהַחוֹתֵם בַּתּוֹרָה, מְבָרֵךְ לְפָנֶיהָ וּלְאַחֲרֶיהָ. זֶה הַכְּלָל, כָּל שֶׁיֵּשׁ בּוֹ מוּסָף וְאֵינוֹ יוֹם טוֹב, קוֹרִין אַרְבָּעָה. בְּיוֹם טוֹב, חֲמִשָּׁה. בְּיוֹם הַכִּפּוּרִים, שִׁשָּׁה. בְּשַׁבָּת, שִׁבְעָה. אֵין פּוֹחֲתִין מֵהֶן, אֲבָל מוֹסִיפִין עֲלֵיהֶן, וּמַפְטִירִין בַּנָּבִיא. הַפּוֹתֵחַ וְהַחוֹתֵם בַּתּוֹרָה, מְבָרֵךְ לְפָנֶיהָ וּלְאַחֲרֶיהָ:
On New Moon and Chol Hamoed four men read, no less and no more. And there is no haftarah reading in Prophets, [For on New Moon and Chol Hamoed there is, likewise, (the factor of) keeping people from work, essential labor being permitted.] The opener and the concluder in the Torah (reading) recite the opening and the concluding blessing, respectively. This is the rule: Wherever there is mussaf and no yom tov, there are four (readers); on yom tov, there are five; on Yom Kippur, six; on Shabbath, seven. [For whatever occasion has more features than its neighbor has more readers. Therefore, on Rosh Chodesh and Chol Hamoed, where there is a mussaf offering, there are four readers; on yom tov, where work is interdicted, there are five; on Yom Kippur, where there is a punishment of kareth (cutting-off), six; on Shabbath, where there is a punishment of skilah (stoning), seven.] There may be no fewer, but there may be more. And there is a haftarah reading in Prophets. The opener and the concluder in the Torah (reading) recite the opening and the concluding blessing, respectively.