The fact that C is small compared with the initial concentrations of N2 and H2 makes this calculation relatively easy to do. But it implies that very little ammonia is actually produced in the reaction. According to this calculation, only 1% of the nitrogen present initially is converted into ammonia.
What would happen if we add enough N2 to increase the initial concentration by a factor of 10? The reaction can't be at equilibrium any more because there is far too