@Krikor: Thanks for helping me, but I don't want to show the item's path. I just want to show the number of items in a category in the item.php file.
If K2 don't support element to get this number, I think I need to add a querry to get it from database in model.php file and after that, push this element to item.php. But I don't know where to do this querry, just because I fear some sql injections will appear when I access K2 cores. So please let me know, thanks again.