The litter size has, of course, a genetic factor to it. A bitch with a mother that had a large litter has a greater chance of having a large one herself. But it seems like there are more factors that weigh in such as weight at conception, nutritional intake, stress, inbreeding and also the male. Firstly, the more inbred the bitch is the smaller the litter in general. Crossbreds often tend to have a larger fertility rate for example and this information is used in several different animal breeding programs (for example pigs bred for meat are crossed out to receive a larger fertility rate, easier to get the sow pregnant and acquire a larger litters, better health etc.) So this seems to be a negative result from how inbreed some breeds are and also how inbred individual bitches and dogs are.
Secondly I've seen shelties that have given birth to 7 puppies with one male and with another only 3. This can be the result of many things, such as the young unexperienced male having bad swimmers at first (that is one of the reasons you'd rather use an experienced male for the first litter).
Secondly I've seen shelties that have given birth to 7 puppies with one male and with another only 3. This can be the result of many things, such as the young unexperienced male having bad swimmers at first (that is one of the reasons you'd rather use an experienced male for the first litter).