This makes me curious, actually. *goes to check the top posters list*
Vegessene_Nacht_88 has 3,085 posts and five boxes.
cloud has 3,254 posts and six boxes.
So... somewhere between there. Looks like a pretty illogical number.

For seven boxes, you need somewhere between 5,888 and 6,587, and I guess that is the maximum number (that we know of), because no-one has more.
[/has too much time on his hands]