xQuery

for $c in doc("Clients.xml")//CLIENT
let $r:=doc("Rent.xml")//RENT
for $noOfRents in count($r[@CLIENT_ID=$c/@ID])
order by $noOfRents descending
return
    <client>
        {$c/@ID, $c//NAME, $c//SURNAME}
        <Rents>{$noOfRents}</Rents>
    </client>