Both Arioch and the Civ 6 Wiki describe the Kongolese Leader Bonus "
Religious Convert" as:
Kongolese cities can't build holy sites, but they do receive the benefits of the founder beliefs of any religion that has established itself as the majority religion in that city. Receives a free apostle whenever an mbanzaor theater district is built.
That to me says if Kongo has 6 cities, and
each has a different majority religion, Kongo gets 6 lots of founder beliefs. My guess is they mean CIV not CITY. However, if not that's one hell of a bonus.