We provide an extensive and general investigation of the effects on industry performance (profits and social welfare) of exogenously changing the number of firms in a Cournot framework. This amounts to an in-depth exploration of the well-known trade-off between competition and production efficiency. We establish that under scale economies, welfare is maximized by a finite number of firms. Our results shed light on several theoretical issues and policy debates in industrial organization, including the relationship between the Herfindahl index and social welfare, destructive competition and natural monopoly. Our analytical approach combines simplicity with generality.