baker's dozen

Reader's　Digest 2月号の記事 "We Found a FIX HELP, HACKS & HOW TO" からの引用です。
When buying rosesthis Valentine's Day, make it a baker's dozen and keep one bloom for yourself.
Flowers of the same type that are cut at the same time and kept in similar environments mature similarly. So when your flower begins to wilt, you'll know it's time to buy your beloved a new bouguet. Neither your gesture of love nor the thought behind it should die after Feb.14.

"baker's dozen" の表現は前にも見た覚えがありますが、幾つを示しているのか忘れました。　１２本のバラではない事は確かですが、幾つを意味しているのか分かりません。　辞書を見ます。

・Oxford English Dictionary: a group of thirteen (= one more than a dozen, which is twelve), origin This phrase comes from bakers’ old custom of adding one extra loaf to an order of a dozen.
・Vocabulary.com: 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.