Hi.
I've played pieces on piano that have 8-9 fingers at a time plus pedaling. I've played pieces that have just 2 fingers and no pedal that are much harder. So, the number of mental concepts doesn't map directly the number of appendages in use. The muddying factor here is chunking. Familiar chords or riffs will seem like single mental chunks, not groups of individual notes. A good musician will have lots of chunks and so doesn't have to think as hard. Note that no musician has ever been good without a lot of training -- they must be learning *something* during that time!
It seems it would be fair to say that music is harder to play when the decoding of the score generates more chunks. Then, music would be harder when the chunks you know are not enough to keep the decoder's chunk output in check so there's no finger chunk buffer overruns, right?
Andres.