While running the test in `batchedDeletion` "mode", the pre-images are not deleted in doc-by-doc fashion, but rather utilise the efficient multi-deletes functionality.

The number of batches is at least equal to two in the test case as we run the one deletion per collection range.

However, the `BatchedDeleteStage` may perform deletions in multiple batched depending on for example timing. (it will store the fetched documents in a buffer until the  underlying stage EOFs or a timeout is hit, then the deletion is performed)

This is why the number of batches may vary and needs to be accounted in the test.