To get more boxes, choose a different metadata-viewing preset.
If none suit you, create one of your own.
It can be done manually (without a plugin) by editing a text file, but I think it's easiest to use Jeffrey Friedl's "metadata viewing preset editor" plugin.
You choose "metadata viewing preset" from the menu on the left of the panel header. The menu under the panel header is for choosing metadata preset. It's good to understand the distinction, and use correct terminology.. - the "metadata viewing preset" determines which metadata items you'll view, the other (metadata preset) assigns metadata values to photo(s).