How do I count number of nodes?

This is my xml:
---------------
<column-header>
<upper-left rows="1">Business Date</upper-left>
<upper-left rows="1">Entity</upper-left>
<upper-left rows="1">Account Number</upper-left>
<upper-left rows="1">Account Name</upper-left>
<upper-left rows="1">Currency</upper-left>

<mh n="1" cols="1">Total Account Value</mh>
<mh n="2" cols="1">Initial Margin Requirement</mh>
<mh n="3" cols="1">Maintenance Margin Requirement</mh>
<mh n="4" cols="1">Excess</mh>
<mh n="5" cols="1">Excess (USD)</mh>
<mh n="6" cols="1">Excess (GBP)</mh>
</column-header>

I want my xsl to look like:
---------------------------
<TD valign="center" COLSPAN="x"> where x is the number I'm looking for.

Thanks,
-alex :)
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