Because i use my own custom template i work around a little bit and find a solution for you.
In the category template (default.php) around line 106 (vm default template) in your custom template you can place the following code that uses the category model of VM models. I suggest you that solution so there is no need to override core files.
echo $categoryModelkaraoke->countProducts($this->category->virtuemart_category_id );
The first line assign the model and the second one echo the products number for the category. You can work around and place that code where you need it, you can use some html/css to style it as you wish, like place "(", color, size, etc.
$categoryModelkaraoke --> use a variable as you wish, like $CatModc
This code snippet is for category-view.
Hope helps you out and may helps some other users that need it.
If that solution works for you please mark the thread as solved. Also this thread should be under Templating & Layouts. Please correct it.