We study the relation between the number of firms and price-cost margins under price competition with uncertainty about competitors costs. We report an experiment in which two, three, and four identical firms repeatedly interact in this environment. In line with the theoretical prediction, market prices decrease with the number of firms, but on average stay above marginal costs. Pricing is more aggressive than in equilibrium. Absolute and relative surplus increases with the number of firms. Total surplus is close to the equilibrium level, because enhanced consumer surplus through lower prices is counteracted by occasional displacements of the most cost-efficient firm.