... do remember that the greatest concentration of macro-scale organisms are still insects by a substantial degree, which outweigh humans, nevermind out number -- and that's not getting into the real organic rulers of earth that are microorganisms, which outweigh the insects, to say nothing of humans. And insofar as I'm aware, we've had several periods over geographic history of greater amounts of biomass, as well. If everything was ensouled at some point, and those souls remained distinct despite not having something to ensoul in (though the latter, particularly, is questionable.), then presumably there's a lot of free floating souls out there waiting for something to get stuck in.
Though from what a little cursory reminding shows, there's not really a soul, exactly, in hinduism. "Souls" are more or less just particular expressions of Brahman (like, y'know, everything else) and as such are eternal and have always been. No creation point. New souls, to the extent that's actually a thing, are just new expressions of the one truth and whatnot. Individuality as a substantial concept, for souls or otherwise, in hindu metaphysics is a fuzzy thing, honestly, last I checked.
It'd be nice if an actual or former practitioner could chime in, though. Most exposure to it I've personally had was a little low-level academic work and some nice chats with someone that had done some considerably higher level academic work related to hindu philosophy, and that was years ago.