In this paper, considering a cognitive radio (CR) network, we propose a hard combining cooperative sensing scheme that embeds a solution of finding optimal number of users who can participate in user cooperation. The solution to our scheme includes two cases, one when single antenna is used at the CR receiver, and the other, when multiple antennas are employed. Moreover, we have derived the closed-form expression for Bayes risk, which is a measure of probability of error. Bayes risk constitutes false alarm and missed detection probabilities. We have found optimum number of users, who can participate in the fusion scheme, by minimising the probability of error. Our simulation results show the improvement in receiver operating characteristics curve, when optimum number of users are allowed to participate in the fusion scheme. © 2017, Springer Science+Business Media, LLC.