How can I get the number of the nodes of an external file xml

[Qiwen]

I have an external file data.xml.
The structure of the data.xml is like this:
<invoice>
<item>...</item>
<item>...</item>
<item>...</item>
<item>...</item>
</invoice>

How can I get the number of the nodes(item).

I tried
{INCLUDETEXT "data.xml" \c XML \x //invoice/count()\* MERGEFORMAT}
but it doesn't work. Any idea? Thanks a lot!