The equation for Haber ammonia synthesis is: Take the reaction:2N2(g)+3H2(g)⇌2NH3(g)

On the left side of the equilibrium, there are 4 molecules (or moles) of N2 and H2, and on the right side there are only 2 molecules(or moles) of NH3. It is evident that the formation of ammonia is accompanied by a overall decrease in the number of molecules. Pressure is directly proportional to the number of molecules (moles), when other things remain the same..

Therefore, when pressure is increased, the system in equilibrium will tend to neutralise the effect by favouring the direction that is accompanied by a decrease in pressure or the number of molecules. That is, the equilibrium will be shifted towards the right to nullify the effect of increased pressure, as per Le Chatelier's principle.

Conversely, when the number moles of products are greater than that of reactants, increase in pressure will shift the equilibrium to the left. When the number of moles are equal on both sides, increase in pressure will have no effect on the equilibrium.