How many capacitors should I use and why?

The reason to use a small value bypass capacitor across a large value one, is that while in theory for "ideal" capacitors you add them together when connect in parallel, real capacitors work differently with different frequency. Like big value capacitors a good at storing energy, but not good at filtering high frequency noise. So for ideal world you don't practically change total value by adding 10pf to 1micro-f - but each capacitors performs its own function: 1 micro-f will work as a low pass or high pass filter (depending on the L or R connected to it) and 10pf will bypass High F noise.

It is possible when wiring caps in parralel ,to create a "tank" or oscillating circuit.It is usually best to use a large cap paralleled with smaller values to achive the desired value.You will also need to be cautious when by-passing.Some say that a REL 750 pf is enough.Anything above a .01mf is overkill and will alter your high frequencies.Generally speaking,the higher grade caps don' exceed 4 mf.By-passing should be done,keeping in mind the rule of X10.If you are approaching 1/10 the value desired-it may be too much.I am sure that I have oversimplified the situation,but others will surely clarify.

There's no advantage to using multiple capacitors, unlike in a power supply where it could lead to faster discharge times. Bypass capacitors are not used in crossovers; their purpose is to reduce noise that is typically of a much different frequency than that affected by the main capacitor, once again such as in a power supply filter application. Capacitors serve many different purposes in electronics and you are confusing standard practices among different applications.