To further exploit the advantages of the staged combustion aft-injected hybrid rocket motor over the traditional solid-liquid hybrid rocket motor in combustion performance, the effect of the number of oxidizer injectors on the combustion performance of the motor was investigated by three-dimensional numerical simulation based on a designed injector configuration. For the designed injector configuration, by increasing the number of oxidizer injectors, the mixing uniformity between the oxidizer and the primary combustion gas is improved, and the combustion efficiency of the primary combustion gas is accordingly enhanced. In addition, the difficulty of thermal protection for the injector plate is reduced by increasing the number of oxidizer injectors, which is beneficial for further improving the combustion performance of the motor under the current material strength limitations. The research findings can be used as a reference for the design of the injector for the staged combustion aft-injected hybrid rocket motor.