Jewish custom is to read one portion (parasha) of the Torah each week and to complete the entire Torah each year on Simḥat Torah. In Eretz Yisrael, this is the same day as Shemini Atzeret, whereas outside of Eretz Yisrael, Simḥat Torah is the day after Shemini Atzeret.
Even though the standard practice on Yom Tov is to call up five people to the Torah for aliyot, on Simḥat Torah, the parasha, Vezot Haberakha, is divided into seven aliyot. In addition, to honor the completion of the Torah, it is customary to call up every congregant for an aliya. To this end, the first five aliyot are read over and over, until everyone present has been called up, except for the four people who will be called up later: two for the last two aliyot, one for the beginning of Bereishit, and one for maftir.
Le-khatḥila, each time the parasha is repeated, people are called up in the standard order: first a kohen, then a Levite, and then three Yisraelim. If there are more kohanim or Levites to call up, they can be called up for the fourth and fifth aliyot as well as the usual first and second. When possible, they should still be called up in order, meaning, the fourth aliya should be given to the kohen and the fifth to the Levite (MB 135:37). If many kohanim are present but only a few Levites, a kohen should be called up for the first aliya but then a Yisrael may be called up for the second (Meshiv Davar 2:48). Once all kohanim and Levites have been called up, Yisraelim can be called up for all five aliyot.
When there are many congregants, it is customary to split up into multiple minyanim for the Torah reading, in order to shorten the time it takes to give everyone an aliya. Afterward the congregation reassembles for the final aliyot that conclude the Torah.
On Simḥat Torah it is customary to give aliyot to minors under the age of bar mitzvah. The widespread custom is to give an individual aliya to every child who knows how to recite the berakhot and can read along silently with the Torah reader. Minors who are not yet able to do that go up to the bima together with an esteemed member of the community for the aliya called “Kol Ha-ne’arim” (“All the Children”). The accompanying adult recites the berakhot slowly and loudly, and the children repeat each word after him. This is the penultimate aliya, and it begins with the word “me’ona.”
By giving every congregant an aliya, we show that every Jew – young and old, scholar and layperson – has a part in the Torah.
In order to ensure that the Torah is completed each year on Simḥat Torah, there is some built-in flexibility regarding how the parshiyot are broken up. For instance, leap years have four more Shabbatot than regular years, and when Yom Tov coincides with Shabbat, the Torah reading for Yom Tov takes precedence, so there is a varying number of Shabbatot on which the regular Torah reading takes place. Finally, certain parshiyot are linked with certain times of the year: Parashat Beḥukotai is read before Shavu’ot, Parashat Ki Tavo, as noted, is read before Rosh Ha-shana, and so forth. To keep the Torah reading cycle “on schedule,” certain parshiyot are doubled up.