It's probably not a question you've ever turned your mind to, but a team of paleontologists at UC Berkeley decided to try and figure it out.
Imagine a world where Tyrannosaurus Rexes roamed the Earth, gnashing their sharp teeth at any tasty prey, and ruling the roost. But, how many of these vast, very dangerous creatures were there? It turns out, rather a lot.
The team of paleontologists at UC Berkeley reckon that, at any one time, roughly 20,000 adult T-Rexes were most likely roaming North American lands. Extrapolating this out, the team concluded that approximately 2.5 billion T-Rexes were alive during their two and a half million years on Earth.
This marks the first time scientists have computed these numbers. However, their conclusion does come with quite a vast range of figures.
As Charles Marshall, professor at UC Berkeley and lead author of the study, which was published in the journal Science, noted, the number of T-Rexes could vary anywhere from 140 million to 42 billion.
Marshall and his team fed the numbers into computer simulations to minimize the huge variation in figures. And as Marshall pointed out, "Ecological differences result in large variations in population densities for animals with the same physiology and ecological niche."
"Our calculations depend on this relationship for living animals between their body mass and their population density, but the uncertainty in the relationship spans about two orders of magnitude," he said.
Hence the wide number range of potential T-Rexes roaming Earth. Ultimately, the team's best guess lies at 2.5 billion - which is pretty impressive in itself. But imagine if it was actually 42 billion. Scary!