Does an increase in pressure always causes increase in molar concentration of each gaseous substance involved? Question

Statement :
"An increase in pressure (caused by decrease im volume) at equilibrium results in increase in molar concentration of each gaseous susbstance  involved"

This statement is given true, so i tried this reasoning:

Since decrease in volume(V) causes increase in pressure and hence increase in no. of moles(n), so n/V increase  hence molarity increases.

Is this reasoning correct? Please give the real reason for the statement.
Your reasoning is correct as long as there is a decrease in volume. Note that pressure can also increase with temperature and the volume can stay constant. In this case, the molarity will not change. Molarity is not a very useful unit for expressing gas concentrations in equilibria because you have to quote pressure, volume, temperature and of course moles to quote molarity.