How Many Tyrannosaurus Rexes Ever Lived on Earth? Hereâs a New Clue. An estimation of the iconic predatorâs total population can teach us things about dinosaurs that fossils cannot. Computer simulations indicated, with 97.5 percent probability, that there were at least 1,300 adult tyrannosauruses at any given time but not more than 328,000.Credit...Science Photo Library/Science Source April 15, 2021Updated 2:30 p.m. ET Before they were killed off by a meteor that hit Earth 66 million years ago, some 20,000 adults of the iconic ferocious dinosaur predator â Tyrannosaur rex â roamed North America at any given time, researchers have calculated. Thatâs not a precise number, and the correct total could be far lower or higher because of uncertainties like how long they lived, how quickly they grew and matured and the rate of their metabolisms. Still, the research, published on Thursday by the journal Science, opens doors in studying long-extinct dinosaurs beyond what can be gleaned from individual fossils.