The required n attribute is the number of plan elements in the list.
Source
<xs:element name="Steps" type="UnnumberedPlanElementsType"><xs:annotation><xs:documentation>The Steps element is a list of plan elements without numbers.</xs:documentation></xs:annotation></xs:element>
The UnorderedActionGroupType defines an action group for which all of the steps it contains should be executed, but any order of executing them will work, and no particular order of execution is required.
<xs:complexType name="UnorderedActionGroupType"><xs:annotation><xs:documentation>The UnorderedActionGroupType defines an action group for which all of the steps it contains should be executed, but any order of executing them will work, and no particular order of execution is required.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ActionGroupBaseType"><xs:sequence><xs:element name="Steps" type="UnnumberedPlanElementsType"><xs:annotation><xs:documentation>The Steps element is a list of plan elements without numbers.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:extension></xs:complexContent></xs:complexType>