I need to load 6.6 billion bigrams into a collection but I can\'t find any information on the best way to do this.
Loading that many documents onto a single primary key
You can absolutely shard data in MongoDB (which partitions across N servers on the shard key). In fact, that's one of it's core strengths. There is no need to do that in your application.
For most use cases, I would strongly recommend doing that for 6.6 billion documents. In my experience, MongoDB performs better with a number of mid-range servers rather than one large one.