It's not common, but it can happen! This is called a singleton litter, and it can occur for various reasons:
* Few eggs released: The female dog might have released only one egg during ovulation.
* Embryo loss: Multiple embryos may have formed, but some didn't develop or were reabsorbed.
* Uterine factors: There might be issues with the uterus that prevented multiple embryos from implanting successfully.
While it's less frequent, it's definitely possible for a dog to have just one puppy.