Residential solid oxide fuel cell cogeneration systems (R-FCGSs) have high generating efficiencies; however, they must be operated continuously because of their long warm-up times. Moreover, a reverse power flow from a residential cogeneration system to a commercial electric power system is not permitted in Japan. Because of these restrictions, it is considered that the R-FCGSs may not fully achieve their potential energy-saving effects in Japan. In order to improve the energy-saving effect of the R-FCGSs, the authors have been focusing on a power interchange operation using multiple R-FCGSs (IC) installed at residences in a housing complex as an application of a microgrid. In this operation, the electric power generated by the R-FCGSs is shared among the residences in the housing complex with no reverse power flow so that the electric load factor of the R-FCGSs may increase. This paper discusses the effect of increasing the number of the R-FCGSs involved in the IC on energy saving by conducting optimal operational planning based on mixed-integer linear programming. The numerical analyses for various numbers of target R-FCGSs, with a maximum of 20, clarify that the energy-saving effect of introducing the IC is not correlated with increasing the number of target R-FCGSs, but generally dominated by the total heat to power demand ratio and hourly variations in the electric power demand of the residences. Furthermore, it is revealed that for any number of target R-FCGSs, the IC has an advantage in the energy saving over a stand-alone operation of individual R-FCGSs without the power interchange.