Here are some options you could use, depending on the context:
General:
* Group of benches
* Set of benches
* Collection of benches
More specific:
* Row of benches (if they're arranged linearly)
* Circle of benches (if they're arranged in a circle)
* Tier of benches (if they're arranged in levels)
* Bank of benches (if they're in a line along a wall or a path)
You could also be more creative and use terms like:
* Benchery
* Benchescape
* Bencharium
Ultimately, the best collective noun will depend on the specific situation and the desired effect.