What is the collective noun used for cotton?

There isn't a specific collective noun widely used for cotton. You could use more general terms like:

* A bale of cotton (This is the most common way to refer to a large amount of cotton, as it's how it's usually packaged and transported)

* A field of cotton (This refers to a large expanse of cotton plants)

* A cloud of cotton (This is a metaphorical way to describe a large amount of cotton fibers, especially when they are fluffy or white)

You could also be creative and use a descriptive phrase like a mountain of cotton or a sea of cotton.