Muenchian grouping, counting number of groups

Am using muenchian grouping to group subjets and grades as per students. I can get count for number of subjects opted by each student. But I could not get count of number of students.

Input XML

<data>
  <school>MIT Kindergarden</school>
  <year>2013</year>
  <batch>B</batch>
  <result>
    <student>ABC</student>
    <id>001</id>
    <subject>ALG</subject>
    <grade>C</grade>
  </result>
  <result>
    <student>ABC</student>
    <id>001</id>
    <subject>HIS</subject>
    <grade>B</grade>
  </result>
  <result>
    <student>XYZ</student>
    <id>002</id>
    <subject>ALG</subject>
    <grade>C</grade>
  </result>
  <result>
    <student>XYZ</student>
    <id>002</id>
    <subject>ALG</subject>
    <grade>A</grade>
  </result>
</data>

XSLT

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl">

  <xsl:output method="xml" indent="yes"/>

  <xsl:key name="group-by-id" match="data/result" use="id" />

  <xsl:template match="/">
    <schoolresult>
      <schoolname><xsl:value-of select="data/school" /></schoolname>
      <resultyear><xsl:value-of select="data/year" /></resultyear>
      <batch><xsl:value-of select="data/batch" /></batch>
      <totalstudent>[Could not find]</totalstudent>
      <xsl:for-each select="data/result[generate-id() = generate-id(key('group-by-id', id)[1])]">
        <student>
          <info>
            <name><xsl:value-of select="student" /></name>
            <id><xsl:value-of select="id" /></id>
            <subjectsopted>
              <xsl:value-of select="count(. | key('group-by-id', id))" />
            </subjectsopted>
          </info>
          <xsl:for-each select="key('group-by-id', id)">
            <subject>
              <name><xsl:value-of select="subject" /></name>
              <grade><xsl:value-of select="grade" /></grade>
            </subject>
          </xsl:for-each>
        </student>
      </xsl:for-each>
    </schoolresult>
  </xsl:template>
</xsl:stylesheet>

Output XML

<schoolresult>
  <schoolname>MIT Kindergarden</schoolname>
  <resultyear>2013</resultyear>
  <batch>B</batch>
  <studentscount>[Count not find]</studentscount>
  <student>
    <info>
      <name>ABC</name>
      <id>001</id>
      <subjectsopted>2</subjectsopted>
    </info>
    <subject>
      <name>ALG</name>
      <grade>C</grade>
    </subject>
    <subject>
      <name>HIS</name>
      <grade>B</grade>
    </subject>
  </student>
  <student>
    <info>
      <name>XYZ</name>
      <id>002</id>
      <subjectsopted>2</subjectsopted>
    </info>
    <subject>
      <name>ALG</name>
      <grade>C</grade>
    </subject>
    <subject>
      <name>ALG</name>
      <grade>A</grade>
    </subject>
  </student>
</schoolresult>

If you apply this XSLT to your source you will also get the students counted:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="msxsl">

<xsl:output method="xml" indent="yes"/>

<xsl:key name="group-by-id" match="data/result" use="id" />

<xsl:template match="/">
    <schoolresult>
        <schoolname><xsl:value-of select="data/school" /></schoolname>
        <resultyear><xsl:value-of select="data/year" /></resultyear>
        <batch><xsl:value-of select="data/batch" /></batch>
        <totalstudent>
            <xsl:variable name="studentNumber" select="data/result[generate-id() = generate-id(key('group-by-id', id)[1])]"/>
            <xsl:value-of select="count($studentNumber)"/>
        </totalstudent>
        <xsl:for-each select="data/result[generate-id() = generate-id(key('group-by-id', id)[1])]">
            <student>
                <info>
                    <name><xsl:value-of select="student" /></name>
                    <id><xsl:value-of select="id" /></id>
                    <subjectsopted>
                        <xsl:value-of select="count(. | key('group-by-id', id))" />
                    </subjectsopted>
                </info>
                <xsl:for-each select="key('group-by-id', id)">
                    <subject>
                        <name><xsl:value-of select="subject" /></name>
                        <grade><xsl:value-of select="grade" /></grade>
                    </subject>
                </xsl:for-each>
            </student>
        </xsl:for-each>
    </schoolresult>
</xsl:template>
</xsl:stylesheet>

The output looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<schoolresult>
<schoolname>MIT Kindergarden</schoolname>
<resultyear>2013</resultyear>
<batch>B</batch>
<totalstudent>2</totalstudent>
<student>
    <info>
        <name>ABC</name>
        <id>001</id>
        <subjectsopted>2</subjectsopted>
    </info>
    <subject>
        <name>ALG</name>
        <grade>C</grade>
    </subject>
    <subject>
        <name>HIS</name>
        <grade>B</grade>
    </subject>
</student>
<student>
    <info>
        <name>XYZ</name>
        <id>002</id>
        <subjectsopted>2</subjectsopted>
    </info>
    <subject>
        <name>ALG</name>
        <grade>C</grade>
    </subject>
    <subject>
        <name>ALG</name>
        <grade>A</grade>
    </subject>
</student>
</schoolresult>

All I did was added a variable studentNumber using your already defined key and counting that:

    <totalstudent>
            <xsl:variable name="studentNumber" select="data/result[generate-id() = generate-id(key('group-by-id', id)[1])]"/>
            <xsl:value-of select="count($studentNumber)"/>
        </totalstudent>