how to get selected rows number in a multiselect datatable primefaces

hi :) I have a primefaces datatable with rows selction using checkbox , and i don't know how to get the selected rows numbers here is my datatable:

<p:dataTable id="tabJob" var="contenuJob" value="#{templateBean.contenuJob}" selection="#{templateBean.selectedRows}" rowKey="#{templateBean.idJob}"  paginator="true"
                     paginatorTemplate="{CurrentPageReport}  {FirstPageLink} {PreviousPageLink} {PageLinks} {NextPageLink} {LastPageLink} {RowsPerPageDropdown}" 
                     rowsPerPageTemplate="5,10,15" >

       <f:facet name="header">
        Selectionner les lignes
         </f:facet>
        <p:column selectionMode="multiple"  style="width:16px;text-align:center"/>
        <p:columns value="#{templateBean.colonnes}" var="colonnes" columnIndexVar="ind" sortBy="#{templtaBean.colonnes}">
                      <f:facet name="header">
                              <h:outputText value="#{colonnes}"/>
                      </f:facet>#{contenuJob[ind]}
         </p:columns>
                    </p:dataTable>


            <p:commandButton id="valider" value="valider"  action="#{templateBean.valider}"  />

can someone help me please ? do i have to use ajax events or what ?