"Plastic" can be used as both a count noun and a non-count noun, depending on the context in which it is used.
As a count noun:
I need to buy two plastic containers.
The house is made of plastic panels.
As a non-count noun:
We need to reduce our use of plastic.
I accidentally dropped hot plastic on the floor.
In the non-count noun form, "plastic" is used to refer to the material itself, rather than individual units of the material.
