In the long run, firms in monopolistic competition will produce at the quantity where marginal cost (MC) equals marginal revenue (MR), and this quantity corresponds to the efficient scale.
In the long-run equilibrium, the quantity a firm produces is at the minimum point of its average total cost (ATC) curve, which represents the efficient scale.
In monopolistic competition, firms produce at a quantity where price is greater than marginal cost (P > MC). This implies a markup on marginal cost, and it's a characteristic of monopolistic competition.
Monopolistically competitive markets may not be socially efficient. There is a markup on price over marginal cost, and the quantity produced might not be the socially optimal quantity. This inefficiency is due to product differentiation and the lack of perfect competition.