The demonstrations and crackdowns happened in the largest public squares of China's largest cities, in front of thousands of eyewitnesses, and the dead had friends and families who lived to tell about it. So while we don't have exact numbers, we do have reasonably accurate ballpark estimates. For Tiananmen, the best guess is on the order of a thousand dead, with high end 2x that and the official government figures 200-300.
Also, china has many cities. If in the biggest incident there were up to 2000 dead, then it all could easily add up to ten thousands. (But like I said, I don't claim to know)