While "batch" is a collective noun for things produced together, such as a batch of cookies or a batch of students, it doesn't apply to constables.
Here are some collective nouns that could be used for constables:
* Force: A police force
* Squad: A squad of constables
* Unit: A police unit
* Team: A team of constables
* Group: A group of constables
The best choice will depend on the specific context and the size of the group.