did a little experiment...
using the Pads catagory, as it has the most presets, and these were likely to be the most complex
Pads - 69 Presets in total
Layers Used
1- 25
2 - 13
3 - 17
4 - 8
5 - 2
6 - 2
7 - 0
8 - 2
i didnt go thru the whole lot to check, but quite a few had repeated layers
interestingly, one of the 8 layer sounds had 8 layers that were the same, with zero detune and zero spread...so 1 layer would have done the same job
when i first tried this, i looked at the bass catagory as a reference, but realised i was looking at D2 sounds, but here are the results...
Layers Used
1 - 28
2 - 12
3 - 7
4 - 1
5 - 1
6 - 0
7 - 0
8 - 0
of course, you would excpect a bass sound to be much simpler than a pad sound, but layer wise, the numbers are quite similar
in conclusion, realistically, you could easily get away with 4 layers imo....