We start with this sentence:
The Mundugumors of New Guinea extend the incest taboo so far as for each man seven of eight women are ineligible marriage partners.
That rather awkward wording amount to saying this:
The Mundugumors of New Guinea maintain an incest taboo that extends so far that, for any Mundugumor man, seven of eight Mundugumor women are ineligible as marriage partners.
The OP wants to understand the sense of the wording "for any Mundugumor man, seven of eight Mundugumor women are ineligible as marriage partners." The problem for the prospective groom is that he is so closely related (by common ancestry) to 87.5% of the prospective brides in his area (that is, to seven out of every eight women who may themselves be looking for husbands) that marrying any woman from the 87.5% group would violate the Mundugumor culture's incest taboo.
The expression "seven out of eight women" does not imply that the man will marry eight women and that all but one of them must be distantly enough related to him that those marriages do not run afoul of the incest taboo, but that the eighth one can violate it because all the other marriages are not abominations. No.
The expression means that the man must find a wife from among the 12.5% of the female population that he is eligible to marry even when the culture's incest taboo is deployed to its fullest extent. He doesn't marry 12.5% of all Mundugumor women; he doesn't marry one out of every eight Mundugumor women. He marries one woman, and she must come from the 12.5 percent of Mundugumor women who aren't off-limits to him owing to the culture's incest taboo.
Note that the women who fall into the 87.5% that are unacceptable as partners and the women who fall into the 12.5% that are culturally acceptable as brides will differ from one prospective groom to the next—because each person in the society has a different position in the genealogical pyramid that everyone must take into account to avoid a marriage to someone who is too closely related.