Here's why:
* Reflections are not a single entity. They are individual images created by light bouncing off a surface.
* Collective nouns are used for groups of similar things. We have "a flock of birds" or "a herd of elephants," but reflections don't form a group in that sense.
You could use descriptive phrases to convey the idea of multiple reflections:
* A shimmering array of reflections
* A kaleidoscope of mirrored images
* A myriad of reflected forms
Ultimately, the best way to describe a collection of reflections depends on the specific context and what you want to emphasize.