You can always pass the empty document in the method like this

db.collection.CountDocumentsAsync(new BsonDocument());

db.collection.count()

In C# you can use Count() on the cursor of your collection.

Try as below:

var mongo = new Mongo();
mongo.Connect();
var db = mongo.GetDatabase("DatabaseName");
var collection = db.GetCollection<Product>();
var totalCount= collection.Count();

you can check in the following URL this url