Using the c# driver for MongoDB I can easily construct a query against which I can then add SetSkip() and SetLimit() parameters to constrict the result set to a certain size. >
You can do it like this:
var server = MongoServer.Create("mongodb://localhost:27020");
var database = server.GetDatabase("someDb");
var collection = database.GetCollection("item");
var cursor = collection.Find(Query.EQ("FieldToMatch" : "ValueToMatch"));
var count = cursor.Count();
Some notes: