The Shulhan Aruch (O.H. 282:1) writes that one is permitted to add more Aliyot than the requisite seven on Shabbat. The Rama adds (ibid. and ibid:2) that one may add more Aliyot on Yom Tov as well but in Ashkenazic lands the custom was to not do so on Yom Tov or Shabbat. He adds that the custom was to be lenient, however, on Simhat Torah, during which many people are called up to the Torah. Indeed the Ashkenazic custom is to never add Aliyot, except for Simhat Torah, whereas the Sephardic custom is to permit adding Aliyot on Shabbat and the holidays.
When more than the requisite number of Olim are called up, there is a disagreement among the Rishonim as to whether the same portion may be read multiple times. The Rivash (Shu’t HaRivash, § 84) says that it is permissible to call up several people and to repeat the same Aliya for each one. On the other hand, the Rashbatz/Tashbatz (Shu’t HaTashbatz, vol. II, § 70) writes that it is not permitted for portions to be repeated by several Olim since each extra Aliya would be considered an unnecessary blessing. The Shulhan Aruch (ibid:2) rules like the Rivash and says there is no issue in calling up many Olim and reading the same portion many times, and this is the common custom. The HIDA (Birke Yosef 282:3) writes that although the Halacha follows the Rivash, pious people should be strict and not repeat portions.
One way to call up many Olim without repeating portions is to simply divide up the Parasha into more than seven or five Aliyot on Shabbat and Yom Tov, respectively. As long as each Aliya contains at least three verses, the Parasha can be divided into more Aliyot than normal. Rabbi Ovadia Yosef (Hazon Ovadia, Shabbat, vol. II, pg. 230) writes that even though the Shulhan Aruch writes that it is permitted, one should be strict and not repeat portions.
Nevertheless, Rabbi David Ovadia (Nahagu Ha’am, Hilchot Shabbat) and Rabbi Amram Aburbia (Neitve Am, § 282:4) write the Moroccan custom follows the Shulhan Aruch and permits adding more Olim and repeating the same Aliya in the Parasha. Each extra Aliya is known as Mosif and typically the custom is to repeat the last three or more verses of the sixth Aliya on Shabbat and the fourth Aliya on Yom Tov. Since the repeated Aliyot are not obligatory and congregants tend to lose focus, when it is time to read the final Aliya, the Gabbai of the synagogue customarily calls out “Hovat Hayom” (lit. “today’s obligation”) to announce that this final Aliya is obligatory and that everyone should pay attention. Furthermore, the Gabbai calls up the final Aliya by saying “Shevi’i/Hamishi Vehu Mashlim” (lit. “seventh/fifth [Aliya] and he concludes [the reading of the Torah]”).
Summary: The Moroccan custom is to permit calling up more than the requisite number of Aliyot on Shabbat and Yom Tov, and to permit repeating portions in the Parasha when more than the minimum number of Olim are called up.