One jar may not be enough to get improvement on mileage. But you may have noticed some improvement in the smoothness of the engine, the power and the emissions. Generally, ideal is 1 jar for every 1 to 1.5 liters of engine displacement, and never less than 2. But this is just an estimate, and all cars pose their own special challenges. At this point, we don't know if there's such a thing as too much HHO for a gasoline-powered car. There is some evidence that there is such a point in diesels, but you have to find that by experimentation.

Everyone should do 6 cells, if you can arrange to do that.

For a multi-cell system, connect the cells in pairs - in series electrically, so each cell gets 6 volts (in a 12 volt system). This lowers the possibility of the jars and wires getting hot. Also, look at the new method of connecting the multi-cell system, with the outputs in parallel.