Your desired XSLT is:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:param name="filt">
<filters>
<ritem relateditemnumber="8901037"/>
<ritem relateditemnumber="8901038"/>
<ritem relateditemnumber="8901039"/>
<ritem relateditemnumber="8901040"/>
</filters>
</xsl:param>
<xsl:template match="/">
<xsl:for-each select="$filt/filters/ritem">
<textarea>
<xsl:value-of select="@relateditemnumber"/>
</textarea>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
to get output:
<textarea>8901037</textarea>
<textarea>8901038</textarea>
<textarea>8901039</textarea>
<textarea>8901040</textarea>