{
    "_id" : 1,
    "UserName" : "1",
    "Description" : "abc",
    "UserCode" : "abc",
    "Email" : "abc@gmail.com",
    "PhoneNo" : 123,
    "Address" : null,
    "Locations" : [ ]
}

var aggregate = collection.Aggregate().Group(new BsonDocument { { "_id", "$_id" }, { "count", new BsonDocument("$sum", 1) } });
var results = aggregate.ToList();