Revelation 6 tells of war (v. 2-4); famine (v. 5-6); sword, hunger, death by disease, and by wild beasts (v. 7-8); saying that the carnage will kill one fourth of the world's population! That reduces the total population to 4,500,000,000. That is not counting the massive earthquake and the falling "stars" which follow (v. 12-13)!
Who knows how many are slain between Revelation 6, and Revelation 9, but we are told that, following months of being tormented by demonic creatures (v. 1-14), one third of the world's remaining population dies when an army of 200,000,000 troops attack from the East (v. 15-19)! That third reduces the total population to 3,000,000,000!
Of course, this is all hypothetical, because the world's population may actually be less at the beginning of the Tribulation. In Revelation 6:17, John wrote, "For the great day of His wrath is come; and who shall be able to stand?" Notice that the day of God's wrath began in Revelation 6:1! That is very significant because, according to Bible's teaching on the Rapture of the Church, declaring it to be prior to the day of the Lord's wrath (Jn. 3:36; 14:1-3; 1 Thes. 4:13-18; 5:9; Rev. 3:10; etc.), millions of born again believers in Christ will have been removed. God has not appointed His adopted children to experience His wrath; Jesus took our place, suffering His Father's wrath when He exchanged His righteousness for our sin (Rom. 1:18; 5:9; 2 Cor. 5:21; 1 Thes. 1:10; etc.)!
Regardless of the gigantic numbers involved, there is one person living today who needs to make a decision: YOU! Jesus came and took the place of all those who would trust in Him. He died for you! He was buried and rose again, according to the Scriptures, to be your Lord and Savior! It is up to you whether or not you are saved from the wrath to come; you can accept Jesus Christ, or you can experience the most horrific seven years of man's history!
If you believe and trust in Jesus, you will escape God's wrath (Rom. 10:9-10)!