Some stats I found.

~~~ Did You Know? ~~~

Number of cats and dogs entering shelters each year: 6-8 million (HSUS estimate)


Number of cats and dogs euthanized by shelters each year: 3-4 million (HSUS estimate)


Number of cats and dogs adopted from shelters each year: 3-4 million (HSUS estimate)


Number of cats and dogs reclaimed by owners from shelters each year:
Between 600,000 and 750,000 -- 30% of dogs and 2-5% of cats entering shelters (HSUS estimate)


Number of animal shelters in the United States:
Between 4,000 and 6,000 (HSUS estimate)


Percentage of dogs in shelters who are purebred: 25% (HSUS estimate)


Average number of litters a fertile cat can produce in one year: 3


Average number of kittens in a feline litter: 4-6
In seven years, one female cat and her offspring can theoretically produce 420,000 cats.


Average number of puppies in a canine litter: 6-10
In six years, one female dog and her offspring can theoretically produce 67,000 dogs.