How can I modify my XSL-FO stylesheet to efficiently place multiple invoices in the same PDF page using Apache FOP in A4 size page

I have an XML document in the format:

 <root> 
   <Invoice>..</Invoice>
   <Invoice>..</Invoice>
   <Invoice>..</Invoice>
   <Invoice>..</Invoice>
 </root>

XSl :

    <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:fo="http://www.w3.org/1999/XSL/Format">
      <xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
    
      <xsl:template match="/">
      
        <fo:root>
          <fo:layout-master-set>
            <fo:simple-page-master master-name="invoice-page" page-width="21cm" margin="0.5cm">
              <fo:region-body margin="1cm" />
              <fo:region-before extent="3cm" />
              <fo:region-after extent="0.5cm" />
            </fo:simple-page-master>
          </fo:layout-master-set>
    
          <xsl:apply-templates select="Download/Invoice_Download"/>
        </fo:root>
      </xsl:template>
    
      <xsl:template match="Invoice_Download" keep-together.within-page="always">
        <fo:page-sequence master-reference="invoice-page" >
          <fo:static-content flow-name="xsl-region-before">
            <fo:block text-align="center" font-size="18pt">
              <xsl:if test="position() = 1">INVOICES</xsl:if>
            </fo:block>
          </fo:static-content>
          <fo:flow flow-name="xsl-region-body">
            ...........
          </fo:flow>
        </fo:page-sequence>
      </xsl:template>
    </xsl:stylesheet>

I constructed and XSL-FO stylesheet for the xml document and it converts to PDF using Apache FOP.The problem is it prints each invoice in a single page rather than placing multiple invoices in the same page as long as the space is available(using A4 size page). Each invoice takes only about 1/3rd page and remaining 2/3rds page is blank. How can I modify my XSL-FO stylesheet such that it uses the space in the page efficiently by adding as many invoices it can fit in without breaking the invoice?

I tried placing this

<fo:page-sequence master-reference="invoice-page" keep-together="always"force-page-count="no-force"> 

but still the output is the same. This is my first time working with XSL-FO and Apache FOP and can't find much about this anywhere.
Any help is appreciated.

Without seeing your whole XSL or at least the relevant parts, normally someone would do something like this:

<xsl:template match="invoice">
    <fo:block-container keep-together.within-page="always">
       <fo:block>
        <xsl:apply-templates/>
       </fo:block>
    </fo:block-container>
</xsl:template>

This would surround each invoice with a block-container that is always kept together within a single page. If two fit, then you would have two, if three fit then three. One that cannot fit in total is moved to the next page because the inside content is always kept together. The only caveat here is that you should be sure that no one invoice is longer than one page. If it is, it will likely cause an empty page and then the keep condition would be broken.

Note: I added an fo:block inside the fo:block-container in case you have non block-level content in the matches inside of invoice.