XML Tutorial/XML Schema/maxOccurs

To specify the maximum number of times an element or group can appear

The minOccurs attribute must be a non-negative integer (0, 1, 2, 3, or higher).
The maxOccurs attribute can be any non-negative integer, or the word unbounded to indicate that the element can appear any number of times.
The default value for both minOccurs and maxOccurs is 1.
The minOccurs and maxOccurs attributes can not be used with globally declared elements.
These attributes can also be used in xsd:sequence, xsd:choice, xsd:all, and in references to named groups.
You can control how many times a given element, sequence, set of choices, unordered group, or named group will appear.
To specify the minimum number of times an element or group can appear: 
 
<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.wbex.ru" xmlns="http://www.wbex.ru"
  elementFormDefault="qualified">
  <xsd:complexType name="animalType">
    <xsd:sequence>
      <xsd:element name="project" type="xsd:string" minOccurs="2" maxOccurs="5" />
      <xsd:element name="population" type="xsd:integer" />
    </xsd:sequence>
  </xsd:complexType>
</xsd:schema>
Another schema,
 
<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.wbex.ru" xmlns="http://www.wbex.ru"
  elementFormDefault="qualified">

  <xsd:complexType name="animalType">
    <xsd:choice minOccurs="0" maxOccurs="unbounded">
      <xsd:element name="sister_name" type="xsd:string" />
      <xsd:element name="brother_name" type="xsd:string" />
    </xsd:choice>
  </xsd:complexType>

</xsd:schema>