In the reaction MgCI2 plus 2KOH Mg(OH)2 plus 2KCI if 3 moles MgCI2 are added to 4 moles KOH what determines how much Mg(OH)2 is made?

The potassium hydroxide is what we call the limiting reactant. 4 moles of potassium hydroxide react with 2 moles of magnesium chloride. The third mole of magnesium chloride is in excess and has no effect.