This paper shows the optimization techniques for the determination of the number of sensors to estimate the flow states on the flow field. The flow states by a state estimator is fed back to input in the flow controller. Usually a series of pressure sensors mounted on the surface is used as a flow measurement device. To minimize the number of sensors, it is necessary to maximize the sensing effect by optimization of their placement with the state estimator point of view. The performance index for optimizing is thus constructed with the estimation errors. The results provide the optimal sensor positions as well as the optimal number sensors.