var expr = customers
  .GroupBy(
    c => c.Country, // keySelector
    c => new { OrdersCount = c.Orders.Count() }, // elementSelector
   (key, elements) => new { // resultSelector
      Key = key,
      Count = elements.Count(),
      OrdersCount = elements.Sum(item => item.OrdersCount) });

foreach (var group in expr) {
  Console.WriteLine("Kl: {0} - Poet: {1}  Poet objednvek: {2}",
    group.Key, group.Count , group.OrdersCount);
}
