Will increasing the amount of one substance in an equilibrium reaction favor the reaction that produces more or less of that substance?
The reaction that is favoured will be the one that uses up some of the added substance.
Le Chatelier's principle tells us this:
When a change is made to a system at equilibrium, the system will shift in such a manner as to counteract the change that was applied to it.
So, if I raise the concentration of a substance in the equilibrium system, the shift in equilibrium will result in a new equilibrium that has consumed some of the added material.
But be careful here! Adding more of a substance is not necessarily the same as raising its concentration. For example, if you add more of a pure liquid or a solid, the concentration is not increased, and there is no shift in equilibrium.