I don't think this is possible but I thought I'd throw it out there. Given this XML:

 <people count="3">
      <person>Bill</person>
      <person>Joe</person>
      <person>Susan</person>
 </people>

Is it possible in an XSD to force the @count attribute value to be the correct count of defined elements (in this case, the person element)? The above example would obviously be correct and the below example would not validate:

 <people count="5">
      <person>Bill</person>
      <person>Joe</person>
      <person>Susan</person>
 </people>