Collective nouns are typically used for groups of animals, people, or things. Since spray is a substance, it doesn't have a specific collective noun in the same way that you might use "flock" for birds or "herd" for cattle.
However, you can use descriptive language to convey the sense of a group of sprays:
* A cloud of spray
* A burst of spray
* A mist of spray
* A shower of spray
These phrases effectively describe the way spray might be dispersed.