In this letter, we obtain a generalized expression for the optimal number of cognitive users ${M}$ for the ${K}$ -out-of- ${M}$ rule that minimizes the Bayes risk at the fusion center over noisy control channels. We show that many existing and new results for optimal ${M}$ are special cases of the proposed solution. Numerical results are presented using energy detector. However, the expressions for optimal ${M}$ obtained in this letter are applicable to any detector used in cooperative spectrum sensing. © 2012 IEEE.