Here are some possibilities, depending on the context:
* Coil (especially if the ropes are wound together)
* Bundle (for a group of ropes tied together)
* Heap (if they are piled together)
* Stack (if they are arranged neatly)
You could also use a more descriptive phrase, like:
* A length of ropes
* A collection of ropes
* A supply of ropes
Ultimately, the best collective noun will depend on the specific situation and the desired effect.