If we want the category list to show how many items are in each category, CATLISTVARS plugin is the one to use.

It's dependent upon which skins we use, we have to edit the template on that skin. Since we use the modified default skin, therefore, the pictures are from the default skin. On the left handside column of the admin screen, we should see the Templates shortcut. Anyhow, first, we need to install the catlistvars from the admin Plugins screen

Watch video in HD fullscreen for better explanation

Go to the Templates, select Default skin index template and scroll down to the Category List Item, add either (<%numitems%> or [ ] whichever brackets you want the number of items to be in.

When it's done, you should see the display as the first pictures with the number of items in the brackets you chose. If they don't show up, something is wrong with the modified skin that you use. I have one that doesn't show and I haven't figured out why yet!

According to the instruction at the plugin page, we need to edit the item template, in our skin templates, there are index and item, both contains the category list item, I tested... ... them both, one with [] and one with (), the index one is the one showed up.

You can try both to see which one works for you.