As I don't have enough information, I can't determine who Mami Ki is in the story. It could be a main character, a supporting character, or even a name given in a metaphorical sense.
Another theme is time - related. Since Homura has the ability to manipulate time in the show, many fanfictions play with this concept in relation to her relationship with Madoka. For example, stories might involve Homura going back in time to save Madoka in different ways or dealing with the consequences of her time - traveling on their relationship.
Rather than a'sexy mami' story, here's a story about a Hindi mother. In a small village in India, there was a mother who was known for her beautiful singing of traditional Hindi songs. She would sing to the children in the village, passing on the cultural heritage.
A great Mami Tomoe fanfic often has a deep understanding of her character. For example, it shows her elegance and grace on the surface but also her loneliness and vulnerability underneath. If it can capture these aspects well, it's likely to be a good one.
The 'mami wata' story often involves a water spirit in African and Afro - diaspora cultures. It's about a being that is part human and part fish or snake - like. These spirits are often associated with beauty, wealth, and power. They can be both benevolent, bestowing good fortune, or malevolent, causing misfortune if not respected.
I think there are several, but the exact number might change as new ones could be released. You might want to check reliable manga databases or official sources for the most accurate count.