A baker's dozen is thirteen of something. If your cat has a baker's dozen of kittens, she has thirteen babies.
You can use the term baker's dozen any time you're talking about thirteen items. You might buy a baker's dozen of stuffed animals for your many cousins, or bake a baker's dozen of chocolate chip cookies. In fact, baking is the origin of baker's dozen — since the sixteenth century, the term has been used to talk about the longstanding custom of bakers adding an extra loaf or muffin or bagel to a customer's order of a dozen.