hello everybody! Is it possible to group_by and order results (grouped_hits) by number of hits per group?

I think what you want here is faceting?

I need to group results by 2 fields (category and brand). And I want to order results by number of items per group. Faceting would create two lists, one grouped by category and the other grouped by brand. Is it correct?

Correct, there's no way to do a group by multiple fields, but order on counts.

thanks