Is it normal for a dog to have one puppy?

Depends. There are numerous factors that can affect this.

Smaller dogs tend to have smaller litters - a healthy German Shepherd may have a litter of ten, while an equally healthy Miniature Schnauzer may only have three. A dog's first litter may be smaller than subsequent litters. If the bitch is ill or malnourished, her reproductive system may abort one or more pups before they come to term. If there was only 1 coupling, or the male was an older dog, there may be a limited number amount of fertilization. If the female is older she may not throw as many as she used to be capable of.

Is it "normal?" It's uncommon, but not unheard of.