From Muppet Wiki

The Old Lady Who Lived In a Nine was an animated segment (and name of the central character in the story) created by Bud Luckey for Sesame Street. Using rhyme, it tells the tale of a woman whose life focused around the number nine (she had nine kids, nine dogs, and nine cats). However, once she discovers she has one more dog, the family packs everything to move into a "10."