“We demonstrate that after more than six decades, estimates of global species richness have failed to converge, remain highly uncertain, and in many cases, are logically inconsistent.”
Source: Authors Caley, Fisher and Mengersen writing in a 2014 paper for Trends in Ecology & Evolution.
The paper looked at the estimates for total land and marine animal species, which varied from 0.5 million to 10 million species.
A 2015 paper in the journal Proceedings of the National Academy of Sciences (using mathematical scaling law methods to predict likely microbial species numbers) found that :
“Using this scaling law combined with the lognormal model of biodiversity, we predict that Earth is home to as many as 1 trillion (1012) microbial species.”
see Scaling laws predict global microbial diversity
Current estimates are that, at best, only a tiny fraction of species on Earth have so far been properly catalogued and described.
A set of 2017 experiments at Stanford found that "Nearly all microbes inside us unknown to science"