Each piezo pickup will output a signal. You can use a larger piezo and get more output. You can also use multiple sensors and connect them in parallel so they act like one big piezo.
Its important to note that the output is a function of AREA not diameter so the larger discs give an exponentially greater output.
You can connect two or three or four or five or six (etcetera).
The benefit of separate sensors is that you tend to get clearer responses from different strings. The best solution is to have a separate sensor for each string. But, that's difficult to do unless you make tiny sensors and can separate the string inputs with a split saddle or some such.
So its merely balancing your options:
- simple installation
- space to install the sensors
- clarity of sound
- output levels
- cost
It turns out most companies have found that 2 or 3 sensors seem to be optimum for an bridge-mounted soundboard transducer pickup.
There is no one answer nor one "rule of thumb".
You would need much smaller discs to fit six. That would give you SOME clearer separation of strings but at reduced output.
A single disc would give good output but it would be pretty big and wouldnt pickup all the strings evenly.
So, spreading to or three discs out under the strings is kinda about the optimum solution. They are far enough apart to separate string vibrations, while still giving high enough output and not taking up too much space.
I recently had a K&K where my guitar was less responsive on the treble side so I added a fourth disc and it brightened up the tone while adding a little output level.
so.....in a nutshell: it depends.