Q:

What kind of card sorting is the best?

I have an online grocery website, it includes different categories and subcategories, I want to test the the usability of categories tree, in this case closed card sorting or open card sorting is the most suitable? and how many people should participate ? 

A:

Open or closed card sorting is the popular ones, but there is another interesting technique that put less mental load on participants, the Delphi method:

In this approach, the first participant creates an initial sort, and then the remaining participants review and modify that sort. This imposes a much lower mental load on the remaining participants, rendering a sort of 200+ items a much more tractable proposition (in theory).

Ref: Card Sorting using the Delphi Method
It's an alternative supported by Donna Spencer who wrote the Card Sorting Book.

