Using rev.categories.xml.config:
There's a field named NumObjects within EntityHelpers that contains the number of products per entity.
To add it just:
Code:
<a href="{concat('c-',EntityID,'-',SEName,'.aspx')}">
<xsl:if test="EntityID = $CategoryID or descendant::Entity/EntityID = $CategoryID">
<xsl:attribute name="style">font-weight:bold</xsl:attribute>
</xsl:if>
<xsl:value-of select="$eName"/>
<!--output the number of products for this category-->
<xsl:value-of select="concat(' (', NumObjects, ')')" />
</a>
This field, along with other entityhelper values are cached upon site startup, so if you add another product, you'll have to reset cache for it to update accordingly.