Normally it isn't as long as no strong electric fields affect the devices from the outside. If for some reason the effectiveness of a device should decrease or if the device should not work any longer it can be recharged at any time. In the case of critical industrial applications, where a lot of strain is on the rings, Merus is replacing as a part of the contract the rings once a year. So it is assured Merus is working all the time with the highest possible performance.
For home use in small accomodation units normally one Ring, which is installed around the main water pipe, is sufficient. For industrial applications, it depends on the task to treat e.g one pump one Merus ring is enough, to treat a big cooling loop with hundreds of heat exchangers, accordingly more rings are needed.
No, they're not. Merus can only influence the substances dissolved in water. Particles contained in water are still in it when it has passed the Merus device. But these can mostly be removed using filters.