Does anyone know?

The thirteenth one is a little something extra that the baker throws in for customer goodwill.

In New Orleans (and maybe France and French-speaking countries?), it's called lagniappe.

Thanks, Mary Anna.

I know this word, actually.

It means, "a little something extra" and, at least, I don't think it applies exclusively to baked goods.

Please correct me if I'm wrong.

I first heard it in New Orleans years ago.

No, it doesn't only apply to baked goods, but I've sure gotten a 13th baked good at bakeries there.

I do think "baker's dozen" in English started at bakeries but has spread out to other forms of commercial generosity.