I know you can buy things to make you have more pet slots, but how many can you have without spending extra money?

Five pets. xD

Buying a gold account gives you five extra, and buying the mythical plushies gives you extra ones as well. <<

You can have 5 pets total on a regular account, free of charge.

You can get 10 with an activated Gold Account, and 20+ using a GA coupled with Mystical Montre or Donadak plushies or the pet slot totem from Loyalty Boxes that are given out free every year starting with your second year on the site.

5 pets for a basic account. You can have up to 10 without buying extra items if you get a gold account, the 1 week ones can be pretty cheap sometimes.

Ah, thankyall very much:)