How many capacitors are needed for a step down circuit?
Input capacitors

Ideally, the input to the regulator has 0 impedance.  That's not possible, of course, especially if there are long wires between the regulator input and wherever the input voltage is coming from.  The capacitors guarantee a low impedance at the frequencies where this really matters.

Since 0 impedance is ideal, infinite capacitance would be great.  The datasheet gives you guidance of how much capacitance is good enough for what the chip needs.

The extra 100 nF is to provide a lower impedance at even higher frequencies than the larger caps are effective at.  The extra 100 nF by itself is inconsequential when added to 20 &micro;F.

Output capacitors

The output of this switcher is pulses of current.  That requires capacitance to smooth out to a reasonably steady voltage.  In this case, the capacitance does two things

It smoothes the pulses to make a flat-enough output voltage.

It is part of the overall system the controller in the regulator is working with.  Both too little and too much capacitance can cause control instability.  Stick to what the datasheet says.



Values towards the higher end of the range will result in less output voltage ripple, but also allow less additional capacitance to be distributed among the consumers of the output voltage.

Multiple capacitors

Often multiple identical capacitors are used in parallel on both the input and output of such switching regulators.  The purpose is to get more capacitance than a single part of the right type can provide.  These capacitances need to be low ESR (equivalent series resistance) to the point that they pretty much need to be ceramic with today's technology.  Ceramic caps can only be so big before mechanical problems make them unfeasible.  Multiple caps in parallel allow for higher capacitance at the same voltage.

If a new technology came along that could fit the combined capacitance into a single part without mechanical problems or higher ESR, then it would be fine to use such caps.  The technology tradeoff keeps changing, within the lifetime of many datasheets.  Look around.  Perhaps you can get the total capacitance with the right specifications in a single package today.