Does anyone know how to activate this achievement? I have 11 pets but none of them are counted towards the achievement.

Have you been to your Pets page?

I've been to my pets page, their individual pages and their profiles. None of them have done anything. :( I don't understand it.

It's your pets page that triggers it, and it's working for me. I just tested by adopting + abandoning another pet

Oh ok. Right now none of my pets are counting towards the achievement and that makes sense if you need a minimum of 20 pets.