There can be many but one at a time.
there are many layers
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.
Why are there so many layers in the atmosphere
30 minutes of cardio a day.
how many tissue layers do the phylum arthropoda have
100 layers
10 layers
The cantaloupe has 4 layers.
7 layers
The bed has three 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).