It depends on what type of swimming pool you have, you need to consider its shape and also the swimming pool lights that you are going to install.  No matter the swimming pool lights are surface-mounted or recessed mounted, you need to place them at an equal distance from one to another, you should ensure all the swimming pool lights in the pool distribute even lighting across the water. If your pool is curved then you may need to take into account the beam spread of the light and the angle with which the light will be projected.