This entry isn't about a single prophecy, but rather a series of individual prophecies covering judgments, war, famine, plagues and other events that will happen both during the Tribulation and the Pre-Tribulation period, starting from approximately around the time of the Rapture forward to the second coming in which a massive amount of death and world depopulation occurs.  At the start of this period the world population will potentially be as high as 7.5 billion.  By the time of the second coming, when this will all end, there will be less than 500 million people left on Earth, possibly as few as a hundred million, a reduction of up to 98% of the Earth's population that existed at the time of the rapture.  But at the absolute most, no more than 10% of those alive at the time of the rapture will still remain at the end.

EDIT (2023-05-09): Setting this back to active since we're about to go through one of what I believe will be several major depopulation events, both prior to, and during the Tribulation.  See this prophecy for more details on one of possibly several pre-tribulation depopulation events: Great Death Will Fall Upon The World