insertMany group of processes can have at most 1000 operations. If a group surpasses this limit, MongoDB will divide the group into smaller groups of 1000 or less. For example, if the queue contains 2000 operations, MongoDB creates 2 groups, each with 1000 operations.
The sizes and grouping workings are internal performance details and are subject to change in future versions. Executing an ordered list of operations on a sharded collection will usually be slower than executing an unordered list since with an ordered list, each operation must wait for the previous operation to finish.