Multi-FPGA systems have received an attention as a computing cluster for multi-access edge computing (MEC). Also, they can process time-critical jobs with their hardwired logic. For this purpose, the static time-division multiplexing (STDM) network is adopted because it enables to predict latency and bandwidth. However, the overall performance of the STDM network depends on the number of time slots. This paper proposes a new mapping tool that optimizes the application mapping so that the number of slots is minimized. Our tool handles multicasts and multi-ejection function which are effective techniques for STDM switches implemented on an FPGA cluster. For applications with all-to-all communication, our experimental results show that the tool reduces the number of time slots by 59-68% with both multicasts and multi-ejection switches.