\useMPlibrary [dum]
\setuppagenumbering[state=]
\setupcaptions[number=no]

\starttext
  \startluacode
    context.setupblank({"10mm"})
    context.setupcombinations({distance="10mm", after=context.blank()})
    context.startplacefigure({title = ""})
      context.startcombination({nx=3, ny=4})
        for i=1, 12 do
         context.startcontent()
           context.externalfigure({dummy},{width = "3cm"})
         context.stopcontent()
        end
      context.stopcombination()
    context.stopplacefigure()
  \stopluacode
\stoptext
