kids? He says back to her, because we didn't want 5? However in the next season, they suddenly introduce an older daughter out of nowhere. And they now have 5 children.
This joke is actually a slight variation from a stand-up routine he did years ago on a video called "Himself", where he says the reason he and his wife have 5 children is because they didn't want to have 6. If you like the show, you'll love the video.
The first episode of The Cosby Show could have been the pilot episode (the episode that decides whether or not future episodes will be made).
After the pilot episode was accepted with open arms, TV producers and executives then take control and do with the show as they see fit sometimes modifying the show's original premise and changing it beyond recognition. We're lucky that adding another child was the most major change.