MongoDB C# driver: Setting consecutive item number during insert

Question

Using mongodbdriver in version 2.10 in a C# project.

I have a document type as follows:

{ 
    "_id" : ObjectId("609720e0fc341e5879c91779"), 
    "number" : "1"
}

Currently when creating a new object, I set the number manually:

var item = new Item();

item.number = getCollection<Item>().CountDocuments(new MongoDB.Bson.BsonDocument()) + 1;
await getCollection<Item>().InsertOneAsync(item);

When executed simultaneously one several machines, this sometimes leads to different documents with the same number, because the count can change between CountDocuments and the insert.

{ 
    "_id" : ObjectId("609720e0fc341e5879c91781"), 
    "number" : "2"
}
{ 
    "_id" : ObjectId("609720e0fc341e5879c91783"), 
    "number" : "2"
}

Is there a way, to set this number on insert in an atomic command like so?

await getCollection<Item>().InsertOneAsync(item).OnInsertSet(item.number = db.collection<Items>.getCount();

Your help would be very much appreciated.