Question: How many women were there?
a. 5
b. 5 or more
c. 6 or more

Logically, it would be "b": not(x<5) -> x>=5, but my guess is "a". ☺

Both #brauchinet and #danepo are right. The "no fewer than" construction means that the speaker is marvelling at the size of the number. Even in societies that tolerate plural marriage, five wives is a large number indeed.

As the Cambridge English-Chinese Dictionary points out, "no fewer than" is used "to show that you consider a number to be surprisingly large [ 不少於，多達 ]." They give this usage example, which may help you if you understand the hanzi: "No fewer than five hundred delegates attended the conference. [ 多達500名代表出席了大會。]"

One could also say that "no fewer than" is a synonym for "at least".

@Objectivesea
Do you mean there were exactly five women and not more than five?

Perhaps should this sentence be tagged ambiguous?

It's not particularly ambiguous, in my opinion. The author asserts that there were at least five women. There may have been six or seven, but most likely it's five. Nevertheless, the author thinks that it's a passing strange occurrence to have so many people claiming a marriage relationship with the man. Such a thing is not a usual event.

Many English textbooks and dictionaries in Japan say that "no fewer than five" means "as many as five", that is, exactly five, which is surprisingly many, but not six or seven. They often advise not to confuse it with "not fewer than five", which means "at least five". Are they wrong?

(Actually, is there an English expression that emphasizes the number while keeping it exact? My dictionary suggests "as much as"(?))

Objectivesea's and tommy_san's analysis of the sentence is correct. If I had said "at least five", I would have meant that I was unable to determine the number.

I don't think that there's a difference between "not fewer than five" and "no fewer than five".