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Since aluminium sulphate is in excess, it implies that sodium hydroxide is the limiting reactant and decides how much product is formed as it gets used up first.

The balanced chemical equation represents ratio in which the chemicals react.

Since $6 mol NaOH$ produces $2molAl(OH)_3$, ie 3 times less, it implies that $2,3$ moles of $NaOH$ will produce $2.3/3=0.767$ moles of $Al(OH)_3$.