To determine the number of projectiles in Future Airspace Window (FAW), we deduced the damage probability formula for the future airspace window with many projectiles configured for each aiming point according to the optimal firing density of the future airspace window, and analyzed the feature of the formula. Based on the formula, the methods for calculating the number of the projectiles under predetermined damage probability or predetermined derivative of damage probability, and the number of aiming-points under predetermined number of projectiles, were provided.The simulation example showed that, the results of damage probability obtained though the formula were nearly the same as that obtained through analytic method. In both cases, the damage probability result of simulation was basically the same as theoretical result, which means that the method has certain feasibility.