I think that i will first explain he basic history of the cathedral itself. It is believed that originally there was a temple of Jupiter on the spot of current cathedral. According to Alain Erlande-Brandenburg there was a basilica build over the former temple in 4th century while Michel Fleury claims a church was build during the reign of king Childebert I (511-558). regardless when the original cathedral was dedicated to Saint Étienne (St. Stephen) and the Virgin Mary. Paris was a much smaller city during the early middle ages, it was mostly confined to the island of Ille de la Cite. Even in the year 1100 Paris probably only had 3000 inhabitants (Colin Jones, Paris: Biography of a City (New York: Viking, 2005), pp.32-37) but during the 12th century is started to grow, specialists estimate that the population of Paris rose from 25.000 inhabitants in 1180, the beginning of the reign of Philip II Augustus , to 50.000 around 1220 (Baldwin, John, Philippe Auguste et son gouvernement - Les fondations du pouvoir royal en France au Moyen Âge, traduit de l’anglais par Béatrice Bonne (préface de Jacques Le Goff), Fayard, 1991).

The cathedral church of Paris was the only parish church in the city until the 12th century. The others had chapel status. In 1182 king Philippe Auguste ordered the expulsion of the Jews from France. Their synagogue, on the very same island, was given to bishop Maurice de Sully who transformed it into a church under the name of Madeleine-en-la-Cité. Maurice de Sully created 12 more parish churches including Saint-Barthélemy, Saint-Jean-le-Rond, Saint-Denis-de-la-Chartre, Sainte-Croix, Saint-Germain-le-Vieux, Saint-Pierre-les-Arcis, Saint-Christophe, Sainte-Marine, and Saint-Pierre-aux-Boeufs. The sacraments of the Eucharist and confession were celebrated regularly in these churches but all baptisms and funerals in the entire city were celebrated exclusively at Notre-Dame. Other places of worship remain chapels like Saint-Symphorien, Saint-Aignan, Saint-Denis-du-Pas. All these churches except the cathedral were destroyed during the French Revolution.

In 1160 bishop Maurice de Sully decided to construct a new cathedral, he demolished the earlier cathedral and decided that the new church should be built in the Gothic style, which had been inaugurated at the royal abbey of Saint Denis in the late 1130s. The cathedral was more or less completed by 1250s and king St. Louis IX wanted to increase its prestige and attract visitors by adding holy relics brought from Constantinople like the Crown of thorns, a nail from the Cross and a sliver of the Cross (however the relics were later taken to Sainte-Chapelle where some of them like the crown remain). The original cathedral had an entire small city enclosed by a wall nearby, where the clerical community of Notre Dame lived and worked.

There was a school from which the university of Paris was born, these teachers and students were among parishioners. In this school future priests were educated (grammar, rhetoric, dialectics, arithmetic, geometry, astronomy, and music were the main subjects) and as a result some of the greatest intellectuals not just in Paris but also in Europe attended services inside and were part of the cathedral chapter. When constructed the Notre Dame cathedral was the largest building made from stone in Europe north of the Alps. It was 125 meters long, with towers 63 meters high, and seats for 1300 worshippers.

The cathedral was attracting royals (on 16 December 1431 Henry VI of England was crowned king of France in Notre-Dame, aged ten, since Reims Cathedral where French kings were usually crowned was being under French control), it had artisans as parishioners and donors (the Parisian goldsmith guild had made regular donations to the cathedral chapter) and it soon became a symbol of France and of the church to the point that the cathedral became a temple to the Cult of Reason and later Cult of Supreme Being during the revolution (to symbolize the desired death of the Catholic church in France) but Napoleon restored it as a church in 1801 and was crowned inside (the interior was redecorated in neoclassical style to symbolize how the Catholic faith is restored but how it need to adapt to new times and leave the middle ages and ancien regime behind).

So in short the cathedral used to be the only real church in Paris back when only few thousand people lived there, as the city grew so did the number of churches. In 2018 the archdiocese of Paris has 106 parishes grouped into 28 territorial deaneries, each comprising an average of 3 or 4 parishes. The deaneries are themselves grouped into 5 vicariates.

Sources :

Baldwin, John, Philippe Auguste et son gouvernement - Les fondations du pouvoir royal en France au Moyen Âge, traduit de l’anglais par Béatrice Bonne (préface de Jacques Le Goff), Fayard, 1991).

The Catholic Church in Paris, sur paris.catholique.fr ,2018

Jean Hubert , "The origins of Notre-Dame de Paris", Revue d'histoire de l'Eglise de France , 1964

Riché, Pierre (1978), Education and Culture in the Barbarian West: From the Sixth through the Eighth Century, Columbia: University of South Carolina Press,

Goyau, Georges. "St. Louis IX." The Catholic Encyclopedia. Vol. 9. New York: Robert Appleton Company, 1910.