This is the standard Torah division by Parasha. The Torah was divided into 54 parashots. Due to variance in the number of weeks in each Hebrew year and not always having the same number of weeks some weeks will combine two parashots into one Parasha for a week.
The original and main institution of kri’at hatorah on Shabbat is to publicly read from the Torah, with at least seven aliyot and and 21 p’sukim (Megillah 21a; 23a). In fact, the original, Torah reading cycle in Eretz Yisrael was three years long. A later institution was made to complete the Torah reading every year.
On Yom Tov days, the reading is divided among 5 Aliyot.