I can't help it, but as a physicist the first thing I look at are orders of magnitude and relations in orders of magnitude. And given the total number of cells

I can't help it, but as a physicist the first thing I look at are orders of magnitude and relations in orders of magnitude. And given the total number of cells in a (human) body (~10^13 according to https://www.ncbi.nlm.nih.gov/pubmed/23829164 https://www.ncbi.nlm.nih.gov/pubmed/23829164) compared to the number of stem cells stated in the article that'd would be 10 orders of magnitude in between. Just talking about red blood cells it's still a staggering 10^12 / 10^3, i.e. 9 orders of magnitude. And for white blood cells it's 10^9/10^3, i.e. 6 orders of magnitude. Next question: Does the number of stem cells scale with body mass? Because if that's the case then I give you the Naked Mole Rat (aka the penis with teeth https://en.wikipedia.org/wiki/Naked_mole-rat https://en.wikipedia.org/wiki/Naked_mole-rat ). The Naked Mole Rat is _THE_ model organism for longevity, completely defying extrapolation of ageing models. The average Naked Mole Rat weighs about 30g, the average human weighs about 75kg. So a Naked Mole Rat hat about 1/2000th the body mass of a human. Assuming a mass proportional stem cell count that would make about 1000 stem cells in total and 0.5 stem cells replenishing blood cells at any given moment. Where does these 20000 and 1000 figures come from in the first place? Given my personal experience in TA-ing students of medicine and biology I don't have a lot of confidence in their calculation abilities. These students manage to get their figures wrong by several orders of magnitude on a regular base. So I'd not be surprised if these figures are grossly underestimated.