(probably) All the times the nickname Sixer is used for Ford by Stan and Bill
it came a long way from only being used twice in the actual show
(book of bill count here)
seven times by stan (EDIT: eight actually forgot about the in-character commentary)
EDIT: "Y'know, I suppose Sixer might be able to make some sorta drill that could get me back down to the dinosaur cave with some real attractions." - Land Before Swine Commentary