We have a general guideline - one jar for every 1 to 1.5 liters of engine displacement. If you have, for example, a 4.6 liter engine, you could go with 4 or 6 jars.
One cell may not produce enough HHO and may get too hot because of the full voltage applied (we actually need less than 12 Volts).
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.
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.
Please see the complete explanation of the Multi-Cell arrangement in Chapter 14 of Book 1, "Multi Cell", or watch DVD 4.