This will depend on differrent factors. For example the flow rate of the pipe or if the water is recirculated. Some systems will be simple like a house or small factory. Other systems will be large and complex distribution systems.
Merus rings are supplied in all pipe sizes and there are also different strength rings for different situations. For example, a ring for bio-fouling will have additional oscillations programmed into it to take care of bio related fouling. Generally speaking one ring will be sufficient for smaller systems, such as a large house or small factory.