Marcia saw an ad on television for ancestry DNA testing
and thought, ”Why not?” She ordered a kit, swabbed
her inner cheek, and returned the kit for analysis. Several
weeks later, she was surprised to learn that she was 17 percent
Native American. An elderly great aunt confirmed that her mother’s family intermarried with members of Native American tribes in the Pacific Northwest in the early twentieth century. To investigate her maternal heritage, Marcia ordered a mitochondrial DNA (mtDNA) test, which confirmed her Native American ancestry. Based on these genetic results, she applied to several Native American tribes for enrollment as a tribal member. She was shocked when she was turned down. In discussions, tribal officials told her that DNA alone is not sufficient to define who is Native American. Tribal standards for enrollment vary, but usually cultural attributes such as knowledge of the language, customs, and history of the tribe are important considerations for enrollment decisions. Marcia was not satisfied and felt that she had a strong case based on biology alone. 

Question: How many great-grandmothers does any individual (such as Marcia) have? How many of them contribute to the mitochondrial DNA that an individual (Marcia) carries?