The seventh is left as an exercise to the reader.

in line with "Teasing apart the assumptions underlying an argument", perhaps the definition initially included a seventh habit, but it was found to be implied by the remaining six and discharged

Isn't there an old quip about how poor mathematicians are at arithmetic? ;)

If there were seven habit, there would probably be someone who would have hoped for an eighth one.

Come on, it's a perfect number!

7: Schematizing things into, for example, lists.