In software-defined networking (SDN), controllers are sinks of information such as network topology collected from switches. Organizations often like to protect their internal network topology and keep their network policies private. We borrow techniques from secure multi-party computation (SMC) to preserve the privacy of policies of SDN controllers about status of routers. On the other hand, the number of controllers is one of the most important concerns in scalability of SMC application in SDNs. To address this issue, we formulate an optimization problem to minimize the number of SDN controllers while considering their reliability in SMC operations. We use Non-Dominated Sorting Genetic Algorithm II (NSGA-II) to determine the optimal number of controllers, and simulate SMC for typical SDNs with this number of controllers. Simulation results show that applying the SMC technique to preserve the privacy of organization policies causes only a little delay in SDNs, which is completely justifiable by the privacy obtained.