It's because there are only thirteen zodiac signs, and the cherubs take the thirteenth.
We're counting ancestors?
Then that's 36 revealed thus far, 48 if you count the pre-scratch trolls' ancestors that have yet to be seen. After all, the pre-scratches may have the same DNA as the ancestors but they grew up in different worlds and thus developed as different characters, similarly to the alpha kids vs the guardians.
Dirk =/= Bro. Meenah =/= The Condesce. Same DNA, but different characters.
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Yes, but how many of those Pokemon actually get important roles in the TV show or the like? For extended periods of time?
Not nearly as many as there are out there.
Plus, the Homestuck world is massive. Hell, there are actually many worlds out there to learn, not just one. You've got the pre-scratch and post-scratch versions of both earth and Alternia/Beforus, that's four, the Inciphisphere, which gets into a ton more if you count each incarnation of the Inciphisphere and way more if you count each planet in it. Then you've got the cherub planet, which I think they mentioned was once a version of earth but still, the Farthest Ring, Dream Bubbles, and, hell, you've even got that Zilly-world from the cherub mythos.
And hell, that's just counting worlds. Homestuck is a massive continuum with a ton to learn about.