What is the best way to evaluate the capacity of a mobile core network with visualization technologies and C/U plane separation using SDN? How can we increase the capacity, especially for accommodating massive M2M/IoT terminals? With increasing demand for cellular networks, enhancing the capacity of the mobile core networks is an urgent issue. In particular, when it comes to accommodating M2M/IoT terminals for cellular networks, the increasing load on the control plane of the mobile core network, as well as user plane, becomes a serious problem. While applying virtualization technologies such as SDN and NFV is one possible solution, there are almost no existing works on numerical or concrete evaluation of such solutions. In this paper, on the basis of mobile core networks with virtualized nodes and C/U plane separation, we first propose a bearer aggregation method for decreasing the control plane load to accommodate massive M2M/IoT terminals. We then show our mathematical analysis of the performance of mobile core networks based on a simple queuing theory. Specifically, we focus on the effect of the node virtualization and C/U plane separation and on the design parameters of the bearer aggregation. The numerical evaluation results show that we can increase the capacity of the mobile core network by up to 32.8% with node virtualization and C/U plane separation, and by an additional 201.4% with bearer aggregation. We also explain that to maintain the performance of the mobile core network, we should carefully determine where the bearer aggregation is applied and when the shared bearer for each UE is determined on the basis of application characteristics and the number of M2M/IoT terminals to be accommodated.