It's a question most palaeontologists don't want to try and answer, because it's so easy to be wrong, by a huge number. Many dinosaur species are known from a single, incomplete specimen. But was that specimen from a population of ten thousand, or ten million? We just don't know. Some environments are really bad at producing fossils, so there could have been animals wandering round in their millions without leaving a trace. Also remember that the "dino era" lasted for 165 million years, and during that time there were incredible differences in the earth's climate. Depending on where you were, you might find it difficult to survive on that basis alone. We're talking ocean temperatures reaching up to 35°C, or about the same as a hot tub. Or oxygen levels at sea level that would be like climbing in the Himalayas today. Don't rely on plant life to help with your survival, as plants evolved just the same as life on land, and fruit and berries simply didn't exist for most of the time of the dinosaurs. Those that did were almost certainly inedible. However, you could always try eating a dinosaur, as their descendants are certainly tasty. Cook it thoroughly though - you don't want to meet too many parasites or microbes that the human immune system has never encountered. To answer the question directly, there was a study that suggested there could have been 20,000 T-Rexes alive in the Northern US at one time. That's roughly the same as the number of wild lions in Africa, or Grizzly Bears in Canada. Maybe the most important thing to remember is that dinosaurs were really just animals, and would likely have reacted to you in the same way as modern animals. Predators would likely leave you alone if they weren't hungry. Herbivores wouldn't care at all if they didn't see you as a threat. You might get chased away if you pose a threat to their offspring, food or territory, but hunting you down after that would just be wasted calories. Answer from Antonio_Malochio on reddit.com

It's a question most palaeontologists don't want to try and answer, because it's so easy to be wrong, by a huge number. Many dinosaur species are known from a single, incomplete specimen. But was that specimen from a population of ten thousand, or ten million? We just don't know. Some environments are really bad at producing fossils, so there could have been animals wandering round in their millions without leaving a trace. Also remember that the "dino era" lasted for 165 million years, and during that time there were incredible differences in the earth's climate. Depending on where you were, you might find it difficult to survive on that basis alone. We're talking ocean temperatures reaching up to 35°C, or about the same as a hot tub. Or oxygen levels at sea level that would be like climbing in the Himalayas today. Don't rely on plant life to help with your survival, as plants evolved just the same as life on land, and fruit and berries simply didn't exist for most of the time of the dinosaurs. Those that did were almost certainly inedible. However, you could always try eating a dinosaur, as their descendants are certainly tasty. Cook it thoroughly though - you don't want to meet too many parasites or microbes that the human immune system has never encountered. To answer the question directly, there was a study that suggested there could have been 20,000 T-Rexes alive in the Northern US at one time. That's roughly the same as the number of wild lions in Africa, or Grizzly Bears in Canada. Maybe the most important thing to remember is that dinosaurs were really just animals, and would likely have reacted to you in the same way as modern animals. Predators would likely leave you alone if they weren't hungry. Herbivores wouldn't care at all if they didn't see you as a threat. You might get chased away if you pose a threat to their offspring, food or territory, but hunting you down after that would just be wasted calories. Answer from Antonio_Malochio on reddit.com