can someone tell me how I can figure out to setup a number of categories per discussion I use on my startpage because I use: index.php?p=categories/discussions.
Well There is CountDiscussions per Category.
If you see categories/all it is using it. Depending on where you want it on the page you can use a theme hook/plugin.
grep is your friend.
I don't see categories/all.
So I can't find a plugin called Count Discussion or similar.
What do you mean with theme hook/plugin?
use the eventi plugin to decide where you want to place the meta
then read this