Megillah, Chapter 4, Mishnah 2. The Mishnah continues to discuss the number of people called up to the Torah on each of the various festivals and occasions.
On Rosh Chodesh and Chol HaMoed:
"B'Roshei Chodashim uv'Chol HaMoed korin arba'ah, ein pochasin meihen v'ein mosifin aleihen" - on Rosh Chodesh and on Chol HaMoed four people are called up; we do not read fewer than this number and we do not add to it. The reason is the same as was explained in the previous Mishnah regarding Mondays and Thursdays: we do not prolong the reading, so as not to hold the community back from going out to work.
On Rosh Chodesh and Chol HaMoed as well there is a concern of neglecting work:
On Rosh Chodesh: there is no prohibition of work at all.
On Chol HaMoed: although work is generally prohibited, work involving a loss - where refraining from it would cause a financial loss - is permitted.
Since there is an element of work on these days, the Sages did not add beyond four people called up.
"V'ein maftirin b'navi" - for that very same reason they did not institute a haftarah from the Prophets on these days.
"HaPoseiach v'hachoseim baTorah mevarech lefaneha ul'achareha" - in the days of the Mishnah, before the Gemara instituted otherwise, the first person to read from the Torah would recite the berachah before it, and the last one would recite the berachah after it. Today, of course, every single person called up recites a berachah both before and after.
This is the rule:
"Zeh haklal: kol sheyeish bo Musaf v'eino Yom Tov - korin arba'ah" - on any day that has a Musaf prayer and nevertheless is not Yom Tov, such as Rosh Chodesh and Chol HaMoed, four people are called up to the Torah.
The number of people called up on Yom Tov, on Yom Kippur and on Shabbos:
"B'Yom Tov chamishah" - work is prohibited on Yom Tov, and therefore five people are called up.
"B'Yom HaKippurim shishah" - on Yom Kippur we add and call up six.
"Uv'Shabbos shivah" - Shabbos is more stringent than Yom Kippur: one who transgresses Yom Kippur intentionally is liable to kareis, whereas one who desecrates Shabbos intentionally is liable to stoning. Because of its added severity, seven are called up on Shabbos.
"Ein pochasin meihen, aval mosifin aleihen" - we do not read fewer than five on Yom Tov, six on Yom Kippur and seven on Shabbos, but it is permitted to add to these numbers. In practice, this addition is our custom on Shabbos alone.
"U'maftirin b'navi" - on these days a haftarah from the Prophets is read, since work is prohibited on them and there is no concern of keeping the community from its work.
"HaPoseiach v'hachoseim baTorah mevarech lefaneha ul'achareha" - as stated, in the time of the Mishnah the one who began the reading would recite the berachah before it and the one who concluded would recite the berachah after it, whereas today each and every person called up recites a berachah both before and after.