Используйте метод Take ()
Dictionary<string, string> colours = new Dictionary<string, string>();
colours.Add("1", "Red");
colours.Add("2", "Black");
colours.Add("3", "Green");
colours.Add("4", "Yellow");
var top2 = colours.Take(2);
foreach (KeyValuePair<string, string> colour in top2)
{
Console.WriteLine("{0}-{1}", colour.Key, colour.Value);
}