הלכה על מגילה 41:13

On Mondays and Thursdays three people are called up (“olim”) to the Torah; four on Rosh Ḥodesh and Ḥol Ha-mo’ed; five on Yom Tov; six on Yom Kippur; and seven on Shabbat (Megilla 21a). Initially, it was customary that each person called up to the Torah would read that section, but as time went on, most congregations designated a Torah reader (ba’al koreh) who would read the Torah for everyone. This is so those who do not know how to read the Torah are not insulted and so the reading itself is especially accurate (Peninei Halakha: Collected Essays I 4:6; II p. 227)