the voltage produced by the lemon cell is dependent on the metals used as the electrodes and can be calculated by using the Electrode Reduction and Oxidation Potentials, but a normal lemon cell using a galvanized nail (zinc) and a piece of copper and you should get a voltage of approximately .9volts.
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You can add pairs of electrodes and connect them in series to increase the voltage as shown in this instructable. The current will still be too low to do much but the voltage will be higher.
The most scientific answer is; why don't you find out?This question is an excellent opportunity for a scientific instructable.You can research the basic idea of how batteries work, then experiment with the types, numbers, sizes, spacing and positioning of electrodes (and lemons). Take photos and notes as you go, write it up, and *presto* - a genuinely educational instructable.In addition, you could investigate the relationship between the voltage output, the current, and the time the lemon provides that power, and you could see if you could design a "standard lemon" to test your electrodes.