There can be many but one at a time.
It is sand. Edit: Over many thousands of years - sediment gets compressed and compacted into layers. The layers form rocks such as slate and coal.
there are many layers
Why are there so many layers in the atmosphere
30 minutes of cardio a day.
how many tissue layers do the phylum arthropoda have
The cantaloupe has 4 layers.
10 layers
7 layers
100 layers
I think what you are talking about is "polyphony". Poly means many, so poly-phony has many sounds. This is as opposed to "monophony" where there is only one musical line going on at a time (mono=one).
It depends how long you live and howw active you are.