28 to 40 habitat species and 9 to 20 exhibit species after Australina pack DLC's arrival?
The only thing we can go off of as now is Planet Coaster, which had around three years of PC support (support meaning regular updates and new content). The variable is that the current DLC model for PlanZo is likely to change. We might see more animals in a future DLC, or less, we just don't know and we can't know. I think ElectricMonk's ballpark figure assumes that the DLC model will remain unchanged (four habitat animals and one exhibit animal per DLC). It's a good guess, but based on Frontier's patterns there's a good chance we'll see change.