> A recent estimate of the global number of prokaryotic species is 2.2–4.3 million [110], down from previous potentially flawed estimates of trillions [111]
From this Nature article: https://www.nature.com/articles/s41396-021-00941-x
From this Nature article: https://www.nature.com/articles/s41396-021-00941-x
A good backgrounder on this topic is "Assessing the global phylum level diversity within the bacterial domain: A review", https://doi.org/10.1016/j.jare.2014.10.005