What is collective noun for bench?

There isn't a widely accepted collective noun specifically for benches. You could use more general terms like:

* A row of benches

* A set of benches

* A group of benches

If you're talking about benches in a specific context, you might be able to use a more descriptive term like:

* A bank of benches (in a park)

* A cluster of benches (around a fountain)

* A line of benches (in a waiting room)

Ultimately, the best choice will depend on the specific situation and your desired level of formality.