Angelus,
As with the days, I do not believe the numbers are literal. The Apocalypse speaks of 7 kings, and then of an 8th, who is the beast (Apoc. 17:10-11). But then there is the beast, and then another beast in Apoc. 13:11 - the 9th?
It also speaks of a beast with 7 heads and 10 horns (Apoc. 17:7), and the 10 horns are "10 kings" (Apoc. 13:12). And it is these 10 kings that make war with the Lamb, and make the harlot "desolate," etc. (Apoc. 17:12-14, 17).
Daniel speaks of 10 horns, and of an 11th that arises, who brings down 3 of them; these horns are also "kings." (Dn 7:24). It is this "11th" horn that "shall crush the saints of the most High: and he shall think himself able to change times and laws, and they shall be delivered into his hand until a time, and times, and half a time." Dan. 7:25. The 11th "king," not the 7th, or rather the 8th, or the 9th(?), or the "10 kings" in concert (Apoc. 17:12-14,17).
I could go on. The numbers, if you take them literally, just don't add up. Not only that, but they are in conflict if literal - being symbolic and figurative, there is no conflict.
As with the "days," I believe we will one day see that these numbers regarding "kings" and "horns" are not literal. They are tropes, symbolically charged, both the images ("kings," "horns") and their number.
DR
DR, some of these issues were settled by the Church Fathers. I don't have time right now to look up each topic, so I will just try to paraphrase what I have come to understand from my reading of the Fathers.
1. The "eighth" is described as "of the seven [kings]." This person is "the False Prophet," who the Church Fathers identify with the Antichrist, the Son of Perdition, the Man of Sin, the Little Horn. Same person.
2. In the passage from Apoc. 17 that I quoted, there are the two Beasts described in more detail in Apoc. chapter 13. The Beast from the Sea has 7 heads and 10 horns. That Beast is the institution of the Vatican City-State/Holy See and its governance structure. The Second Beast described in Apoc. 13, the Beast from the Earth, also called the False Prophet in the Apocalypse, is "the eighth" discussed in Apoc. 17.
These are overlapping descriptions. They are not describing new people. There is only one Antichrist, the single man, that all the Fathers recognize.