Here are some possibilities you could use, depending on the context:
Formal:
* A chime of clocks (emphasizes the sound)
* A collection of clocks (simple and straightforward)
* A battery of clocks (suggests a large, coordinated group)
Informal:
* A tick-tock of clocks (playful, emphasizes the sound)
* A chorus of clocks (suggests a synchronized sound)
* A cluster of clocks (suggests a grouping of clocks)
You could even create your own unique collective noun, like a tick-tocking or a chronometer.
Ultimately, the best choice depends on the specific situation and the effect you want to achieve.