master.xml

<?xml version="1.0" ?>
<ds-count-products-by-category>
  <item handle="compressors">compressors</item>
  <item handle="compressors">compressors</item>
  <item handle="complete-plants">complete-plants</item>
  <item handle="complete-plants">complete-plants</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
  <item handle="chillers">chillers</item>
</ds-count-products-by-category>

master.xsl

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:key name="category" match="ds-count-products-by-category/item" use="@handle"/>

<xsl:template match="/">
	<xsl:value-of select="count(key('category', 'chillers'))"/>
</xsl:template>

</xsl:stylesheet>