You could use a general term like:
* A mass of red hair
* A mane of red hair
* A shock of red hair
* A cascade of red hair
Or, if you want to be more descriptive, you could say:
* A fiery tangle of red hair
* A blazing crown of red hair
* A sea of auburn curls
Ultimately, the best collective noun will depend on the context and the image you want to create.