dog is a viviparous animal so why it give birth to so many young ones??

Dear student, 
Dog is a vivoparous organism. However, the ovary of a female dog releases more than one egg at once. Hence, more than one egg is fertilised and it gives birth to a litter of puppies rather than a single puppy. In humans, there is usually only one egg released per month from one ovary only per menstrual cycle. Hence, humans give birth to only one child usually. 
Regards.