How many / which electrodes should I use as input for an RLD (right leg drive)?
Good questions!  In addition to the common-mode noise reduction, the amplitude of the common mode voltage must remain small in order for the programmable gain amplifiers to operate at high gains without saturating the input stage.  Using a single input from one of the differential input pairs is less ideal, but is still effective because of the low-pass filter being applied to the right-leg drive signal and the fact that the inputs are still differential.  Certainly, closing all of the switches in use gives the best result assuming that you have good connections everywhere.

The purpose of the feature is to provide the ability to de-select individual inputs in the event that there is a loose connection.  Because of the high impedance of the amplifier inputs, an open connection on an input channel, if used to determine the RLD signal, would float and likely interject enough noise and offset to drive all of the differential channels out of their operating range.  The device can detect a loose electrode, allowing you to eliminate its contribution to the RLD drive signal, thus maintaining common mode rejection and preserving the performance of the remaining electrodes.  Choosing a single electrode or electrode pair makes the performance completely dependent on the connection quality of the specific electrode(s) you have chosen.  As an alternate approach to address intermittent or bad connections, you could use this feature to allow the operator to switch between electrode pairs for RLD generation, selecting one which gives the desired result.  The operator could then easily find the loose or intermittent electrode.

Good luck!