NumberedSections

Set the following 2 lines at the beginning of your document:

#set ($context.tocNumbered = true)
#set ($context.tocData = $tdoc.getTOC(2, 6, true))

If you want to have numbered sections across your wiki, one solution is to set these 2 lines inside the xwikivars.vm templates file in the templates/ dir inside the XWiki webapp.