The official maximum number of adults allowed on board may be 678, but I am not sure what it exactly corresponds to.
Boeing 747-400 airport guide suggests that 550 is allowed on main deck - obviously exit limited. And they offer a seatmap with 539 actual seats installed.
But the maximum seat capacity of upper deck is given as 110. Obviously tied to exits rather than actual space available. Since their seatmap shows 85 seats (6 abreast coach seats on upper deck) I do not see where the extra 25 could go.