Doing some rough back-of-the-envelope calculations, I wouldn't be shocked for any value between 100 and 200 billion. You end up with estimates of high single b

Doing some rough back-of-the-envelope calculations, I wouldn't be shocked for any value between 100 and 200 billion. You end up with estimates of high single billions for all humans pre-Neolithic (pre-urban settlement)--the error bars are very high, but it doesn't make a large difference in the grand scheme of things because it's the smallest component. For urban settlement essentially before good, sustained record keeping (think ~10kya to 1kya), you're looking at several tens of billions of people, with several tens of billions more in the era of modern record keeping. Right now, it takes about 6 years or so for a billion babies to be born. (For my part, I think the estimate they give here is on the low end. I don't know the sources they're using, but I've generally observed that estimates of pre-modern demography have generally trended upwards in more recent scholarship.)