master.xml

<data>
    <pants>
        <entry>
            <content>Test 1</content>
        </entry>
        <entry>
            <content>Test 2</content>
        </entry>
        <entry>
            <content>Test 3</content>
        </entry>
        <entry>
            <content>Test 4</content>
        </entry>
        <entry>
            <content>Test 5</content>
        </entry>
        <entry>
            <content>Test 6</content>
        </entry>
        <entry>
            <content>Test 7</content>
        </entry>
        <entry>
            <content>Test 8</content>
        </entry>
        <entry>
            <content>Test 9</content>
        </entry>
        <entry>
            <content>Test 10</content>
        </entry>
        <entry>
            <content>Test 11</content>
        </entry>
        <entry>
            <content>Test 12</content>
        </entry>
        <entry>
            <content>Test 13</content>
        </entry>
        <entry>
            <content>Test 14</content>
        </entry>
    </pants>
</data>

master.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

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

  <xsl:template match="data">
    <xsl:apply-templates select="pants/entry[position() = 1 or position() mod 6 = 1]" mode="row"/>
  </xsl:template>
  
  <xsl:template match="pants/entry" mode="col">
    <li><xsl:value-of select="."/></li>
  </xsl:template>
  
  <xsl:template match="pants/entry" mode="row">
    <aside><ul><xsl:apply-templates select=". | following-sibling::entry[position() &lt; 6]" mode="col"/></ul></aside>
  </xsl:template>
</xsl:stylesheet>