However, you could use "clump" or "cluster" to describe a group of thickets. You could also use "growth", "maze", or "jungle" depending on the specific type and density of the thicket.
For example:
* A clump of thickets lined the riverbank.
* A cluster of thorny thickets made it difficult to pass.
* A dense growth of thickets covered the hillside.
Ultimately, the best collective noun will depend on the context and the specific characteristics of the thickets you're describing.