Bro, animals among the most numerous creatures? Bacteria, Archaea, and viruses are surely more numerous. I bet the same applies to fungi and plants. Oh, and protists, since they are unicellular and have twice the total biomass of animals.
In terms of biomass, animals are barely significant. And they are relatively big. So they surely are less numerous than other groups.
I mean, yes, inaccuracy is another issue here. But I assume "the most numerous" means better than average, or at lest not the worst in its category (unless you actually standardize for the time since divergence, then there are probably some niche microbial taxa, some rare extremophiles, that are actually less numerous; still, surely worse than average).
Yeah, you're right. Some definitions are more limited, but others cover anything that can move of its own accord. I don't think fungi or plants qualify, though.