how to describe the range of this kind of property

Hi all,

	 It really confuse me.
In http://www.w3.org/TR/2004/REC-rdf-primer-20040210, there is a example:
   <rdf:Description rdf:about="http://example.org/courses/6.001">
      <s:students rdf:parseType="Collection">
            <rdf:Description rdf:about="http://example.org/students/Amy"/>
            <rdf:Description rdf:about="http://example.org/students/Mohamed"/>
            <rdf:Description rdf:about="http://example.org/students/Johann"/>
      </s:students>
   </rdf:Description>


My question is how to describe the range of property s:students. Below is a wrong description: <rdfs:range rdf:resource="#person"/>. This description is not compatible with above example. With this range description, I could only use s:students like this:
  <rdf:Description rdf:about="http://example.org/courses/6.001"> <s:students rdf:about="http://example.org/students/Amy"/>
  <rdf:Description rdf:about="http://example.org/courses/6.001"> <s:students rdf:about="http://example.org/students/Mohamed"/>
  <rdf:Description rdf:about="http://example.org/courses/6.001"> <s:students rdf:about="http://example.org/students/Johann"/>


 But actually, I want to say that this course has and only has 3 students, no more, no less.
 How could I do.
 Please help.

Thanks and regards