It depends entirely on the tune, of course. I have a couple of pieces on the desk now that only need two, alternating between them at two points. I'm following the original orchestral score in both cases and the pieces are not that long.
If we take a tune in 'Standard Song Format', ie. A A B A, then we'd really need four, one for each part. If we introduce repeats, like A A B A B A, then five or six. But there may be an intro and and ending part to tack on as well, so eight is easily possible.
More complex pieces will need more, I've used all twelve pistons on the Roland AT900 many times (or 16 on the Yamaha AR100 or EL series), often using one piston more than once. And on occasions, I've used two banks, so a maximum of thirtytwo!
Bottom line is that there is usually that minimum four, but the maximum is as many as is needed to convey the tune to your liking!
It goes without saying that you need to use Registration Memories / Panel Memories / Presets / Pistons (call them what you like!) for this and that you preferably change between them with a foot switch if the hands are busy! What did we do without them? Answer - work ruddy hard, hand registering to get all the necessary changes in.
Now, as for the process.... assuming that you're au fait with the technical aspects of creating and storing the registrations, here are two possibilities (among many!).
1) The gradual build up. Part A: Start 'small' with a solo instrument. If the music asks for two notes, just play one, as I've yet to see a polyphonic trumpet. Part B: Add a second instrument. Part C: Switch to something bigger, piano and strings, maybe. Final Part A: Add more, perhaps add organ to those strings and piano.
2) The pyramid. Parts A, A and B can build up as above, but the Final Part A would drop back down to the starting sound, or perhaps drop to the second sound and then drop to the first.