you can try the following stylesheet:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:ext="http://exslt.org/common"
exclude-result-prefixes="ext">
<xsl:output indent="yes" omit-xml-declaration="yes"/>
<xsl:template match="/">
<xsl:variable name="list">
<articles>
<xsl:copy-of select="descendant::article"/>
</articles>
</xsl:variable>
<xsl:variable name="vPass1" select="ext:node-set($list)"/>
<xsl:apply-templates select="$vPass1/*"/>
</xsl:template>
<xsl:template match="articles">
<xsl:copy>
<xsl:text>Number of articles: </xsl:text><xsl:value-of select="count(article)"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
when applied your input, it produces:
<articles>Number of articles: 4</articles>