The format is given with format attribute. : number « XSLT stylesheet « XML Tutorial

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File: Data.xml
<?xml version="1.0" encoding="utf-8"?>
<data>    
        <n>one</n>
    <n>two</n>
    <n>three</n>
    <n>four</n>
</data>
File: Transform.xslt

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet
      version="1.0"
      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:template match="/">
      <TABLE>
        <xsl:for-each select="//n">
          <TR>
            <TD>
              <xsl:number value="position()" format="001. "/>
              <xsl:value-of select="."/>
            </TD>
          </TR>
        </xsl:for-each>
      </TABLE>
    </xsl:template></xsl:stylesheet>
Output:

<?xml version="1.0" encoding="UTF-8"?><TABLE><TR><TD>001. one</TD></TR><TR><TD>002. two</TD></TR><TR><TD>003. three</TD></TR><TR><TD>004. four</TD></TR></TABLE>