how do u get the massive family achivement when i can only have 10 pets?

, you can increase your pet slots with Mystical Donadak or Montre Plushies, which can be bought from the Cash Shop or users, or Loyalty boxes, which you get every year after two years. You can get a total of 20 pets using the plushies, and more with the boxes (hence my 21 pets).

You have to buy a Mystical Montre Plushie or Mystical Donadak Plushie for extra pet slots. Or when you get the Loyalty Box I think one of the 'gifts' is an extra pet slot.