From here and here from Spy Kids 2:
Gary Giggles is asked his knowledge of cha-cha-cha to which he replies 'Seven steps.' How many steps are there in cha-cha-cha, and what does it mean that he knows seven steps?
From Wiki:
In the American School of Ballroom Dance, the basic step spans two measures of music (frequently counted "one, two, three, four-and, five, six, seven, eight-and" with "five" marking the beginning of the second measure.
So, there are 8 steps, and while Gary doesn't know them all, he knows 7 steps i.e. all but 1 step?