i don't think there is any magic number. it's more about that magic that happens when producers and artists inspire each other.
in an ideal situation the artist and the executive producer should share a vision for the project. who actually makes the indivudual beats shouldn't really matter as long as they all work for the project.
like any creative collaboration, different people can bring a variety of different energy and outcomes to a situation. for some projects, you might want the intensity of the relationship between one producer and one artist for a whole album. for others, you might want the diversity many producers bring.
a lot of commercial projects are heavily influenced by the label's marketing requirements (two club bangers, one with some singing for the ladies, a joint about cars and jewels for the ballers etc), than any artistic vision which is why it's quite rare to find a major release with one producer throughout.
