If you are generating your XSL-FO using XSLT 1.0 then the usual way is
to select all <tr>'s and sort them by the count of their <td>'s, and
then pick the first:

http://www.dpawson.co.uk/xsl/sect2/N7450.html#d9845e601

If you are using XSLT 2.0 then you can use the max() function, eg:

<xsl:variable name="maxCells" select="max(//tr/count(td))"/>

cheers
andrew