When John saw the Great Red Dragon of Rev. 12:3 having Seven Heads and Ten Horns and Seven Crowns upon his Heads, which ONE of the 7 Heads were the 10 Horns attached to?
The heads and horns are both symbolic. We know this because in Rev. 1:1 John wrote that the Lord rendered his message into signs (signified it). This means that the book of Revelation is filled with signs and symbols that describe the main characters and events. The symbolic use of numbers is consistent through out the Bible so we can understand what they mean here.
Seven is the number of supernatural completion and our head is the location of our mind. Seven heads means that the dragon (Satan) has complete supernatural wisdom. Ten stands for totality and horns are a symbol of authority. Having 10 horns means he has total authority on Earth (1 John 5:19). These attributes allow him to indwell the anti-Christ and lead the unbelieving world to think that they can defeat God in battle and take control of His Kingdom here. The seven crowns tell us that through the anti-Christ Satan will have complete rule over Planet Earth for a time. Rev. 17:17 says God will cause the Kings of the Earth agree to this because it suits His purpose.