I recently watched a youtube video where someone wondered the same thing. How many animals should DLCs contain to keep the player base motivated to stay engaged with the game? Are 4 new animals every 70-90 days going to do that? Is that enough for people to stay interested in planet zoo?
He made the argument that it won't, considering that people are likely only interested in 1, maybe 2 new animals out of the 4, which means it's not enough incentive to buy. And with not enough new content coming out, interest in the game will go down. What would be really interesting is DLCs with new animals AND new game mechanics. In other words, stuff that will add new and fresh experiences to the game, like adding flying animals and aviaries, or aquatic animals and tanks.
He didn't think that only 4 new animals every couple of months, along with a new scenario, building theme, and maybe new foliage, will keep players interested.
That said, how many animals I personally think a DLC should have? I think 8 is a nice number that could do the most justice to new themes, without having to exclude iconic animals due to limited slots. If every DLC had 8 new animals, along with the aforementioned new scenario, building theme, and foliage, I think people would be more inclined to buy the DLC and I think people would be burned out less quickly due to more new animals to play with and build for. I personally feel the need for new game mechanics wouldn't be so pressing if there was more choice and variety in the animal roster. DLCs could offer more choice and variety, but not if they come with 3-4 animals.