I hadn't really given it much thought except for the WT artwork in the Revelation Climax
book that many of us JWs had gone over 3 or 4 times. I used to conduct the Book Study in my home during those years. And by the last time I used to keep a non-WT commentary open for comparative reference for myself. (I remember some having a somewhat nervous and/or disapproving look on their faces about that.) And I grew increasingly pensive about the conclusions the WT was putting forth. I distinctly remember by the last time wanting the Book Study out of my house. But I digress . . .
book has the horns distributed somewhat randomly among the seven heads, two each on three of the heads, one each on the other four. The publication takes the ten horns to be representative of all the individual nations in existence at the time of Rev 17's fulfillment. "Ten" being taken as a number meaning earthly completeness.
Much of the beast imagery in Rev 13:1-2 is drawn from Daniel 7. The one difference is that in Rev 13 it is one beast with seven heads connected to it (Rev 13:1) that are then described in terms reminiscent of Dan 7:3-6 (which has separate beasts). But, considering how the ten horns are referred to later in Revelation (Rev 17:3, 12-14, 16-17) and described as, "who have not yet received a kingdom" (Rev 17:12), this reminds me that it is alluding to the ten horns that spring up from the fourth beast in Dan 7:7, 8). (IMO, the small 11th horn that comes up among the ten horns in Daniel 7:8 would be represented by the two-horned lamb in Rev 13:11-12. This one becomes the 7th king of Rev 17:9-10. In Dan 7:11-12, 23-24, 26 this 11th horn is described as having "dominion" ["ruling authority" - NET] just like the other four beasts.)
Given that, IMO, the ten horns would more likely have been seen on this 6th head of the beast in Rev 13:1-2. But having offered that opinion, Revelation itself makes no specific mention of how the ten horns are arranged, making the arrangement of the horns a bit moot as far as understanding Revelation goes. But I definitely believe that the phrase, "who have not yet received a kingdom" makes for the allusion to the ten horns of Dan 7:7-8 rather than WT's 'earthly completeness' idea. WT's earthly completeness' idea for the ten horns falls short because if the ten horns did picture all the individual nations, then, some of them had already "received a kingdom" in past times (e.g. Egypt=Egypt, Babylon=Iraq, Persia=Iran, Greece=Greece, Rome=Italy). (I mention some of this in this thread
My opinion, of course. "Your mileage may vary."
Another digression: Someone told me that they mentioned on the JWTalk forum about the possibility that the ten horns were an allusion to the ten horns of Dan 7:7-8 rather than the 'earthly completeness' idea and that he received a warning from the mods for disagreeing with what "the slave" has revealed. Like Bruno has said, the place sounds like an echo chamber. But I digress . . .