The chemical reaction is:
$4Cu+3S_2->2Cu_2S_3$
So from the left part of the equation, once can say:
For every 4 moles of copper, 3 moles of sulfur is needed.
For every 8 moles of copper, $x$ moles of sulfur are needed.
For every $y$ moles of copper, 9 moles of sulfur are needed.
$x=(3*8)/4=6$ moles of sulfur We have 9, which is enough
$y=(4*9)/3=12$ moles of copper We have 8, which is not enough
So finally, 8 moles of sulfur will react. The reaction will become:
$4Cu+3S_2->2Cu_2S_3$
$zCu+8S_2->wCu_2S_3$
$z=4*8/3=32/3$
$w=2*8/3=16/3$
$32/3Cu+8S_2->16/3Cu_2S_3$
So the maximum number moles that can be formed are:
$16/3=5,bar(33)g$