Depending on what you really want to achieve, you don't really have to make anything "available" in any complex manner. If you want to have the product count displayed as part of your category title, for example, simply update the relevant templates accordingly.
In your example, you may want to change your current templates (depending on where you want this count to be displayed) from something like this:
<h1><?php echo $_category->getName() ?></h1>
To something like this:
<?php echo $this->__('%s %s products in our store!', $_category->getProductCount(), $_category->getName()) ?>
(This would be an example in
Which would then would look something like "59 Cool-Brand products in our store!" as the category title.
The only issue here would be that something like
$_category->getProductCount() is relatively taxing on database performance, but that should not matter to much if you utilize caching.
However, alternatively you could create a new category attribute which holds the product count (or whatever property you are interested in) and calculate this using a cron job which runs once during the night or something similar. You could also add extra logic in there, such as the "round to the tenfold" or something like that. If you need these numbers to be available even quicker, an Observer hooked into the beforeSave event would be an alternative approach as well.
Will this be indexed properly? for search?
If you make it a separate category attribute, it could be used in the search without a big hit on performance. Really depends on what you have in mind exactly.