Dumb Martians Just Sit Nearby Eating Tender Noodles! David, Mary, Joanie, Susan, Nancy, Elizabeth, Tommy, Nicholas.
"Eight is enough" is more natural than either answer using "are", I think. If you use "are", this makes it sound like you elided a noun (in the same way that "Think we can make it?" elides "Do you..."). Using "is" makes the sentence sound more self-contained.
In Portuguese this is right, but "oito é o suficiente" is more common. (with the article)
"Eight is enough" is what most people would say in the UK; I tried to explain it (to myself) when the same text came up in Spanish, and the best I came up with is that we subconsciously think of the "eight objects" as a block. "Do you think sixteen lettuces is enough for the meal after the wedding?" "It 's far too many!"