The documentation describes the maximum number of documents in a single batch operation. This number does not seem to be accurate because a BSONObjectTooLarge error can be thrown when inserting many small documents. An insert command that writing 2861 documents that add up to 16MB in size to a replica set (v4.4, v5.0) causes a BSONObjectTooLarge error to be thrown. For different server versions and topologies, the exact number of documents that can be inserted varies but stays roughly around 2860.