I had to buy at least 3 (or 4 - can't remember) boxes worth (two boxes contained potatos).
The official rules are, "The box must be able to close and not bulge," but it also depends on the cast member who checks you out on how much you can cram into your box.
For instance, when I bought my first box, I put very large pieces (a head, the buzz ride, and teacup) along with many smaller parts in it so it bulged alot and couldn't close. However, the worker who checked me out took out all of my parts from the box and kept rearranging them and stuffing them back into the box so it was barely able to close.
In another example, the cast member took out a few pieces and didn't put them back in so the box would close perfectly.
I bought my parts at different locations depending on what I was doing at the time. I had thought I had all the pieces until I went to the 'Once Upon a Toy' Store. It had all of the parts from the other stores plus more.
I had also tried to take the giant mustache off of the huge Potato Head statue at Pop Century. Unfortunately it didn't come off.
