You can add as much sodium bromide as you like, it’s inert until an oxidizer converts it to bromine. So even if you have 100ppm sodium bromide in the water, if you only add enough bleach or MPS to generate 4-6ppm bromine, then that’s all that it will create. The oxidation/reduction reaction of chlorine and bromine happens quickly and completely - 1 mol of chlorine oxidizes 1 mol of bromide into bromine. The chlorine gets reduced from chlorine to chloride. Chloride is inert and no longer reacts with anything.