The number of batteries in the aircraft model is determined by the CG of the model and the specific load on the wing. You can have a desire to load a large battery capacity, get into the desired CG, with a powerful motor -get a flying brick, the flight time of which will be quickly eaten by a powerful motor. Therefore, with an increase in capacity (weight) batteries to increase the flight time follow the path of reducing the weight of the airframe itself, increasing the load capacity due to a different profile and wing area...The result: increased battery + lightweight airframe = increased flight time with a normal specific load on the wing.