Как отправить push-уведомление более чем на одно устройство (iOS)?

Вы должны поставить в очередь отдельное уведомление для каждого обрабатываемого элемента.
Невозможно установить несколько токенов устройств в одном уведомлении. Возникнет OperationCanceledException, потому что вы это делаете.

Пример: консольное приложение C#

Это предполагает

1. у вас есть действующие сертификаты на производство и разработку
2. вы сохранили несколько токенов устройств в своей базе данных
3. у вас есть уведомление, которое приходит из вашей базы данных

4. Вы используете библиотеку PushSharp.

   using System;
   using System.Collections.Generic;
   using System.Linq;
   using System.Text;
   using PushSharp;
   using PushSharp.Core;
   using PushSharp.Apple;
   using System.IO;
   
   namespace MyNotification
   {
       class Program
       {
           //args may take "true" or "false" to indicate the app is running for
           //development or production (Default = false which means Development)
           static void Main(string[] args)
           {
               bool isProduction = false;
               if (args != null && args.Length == 1)
               {
                   Console.Write(args[0] + Environment.NewLine);
                   bool.TryParse(args[0], out isProduction);
               }
               try
               {
                   //Gets a notification that needs sending from database
                   AppNotification notification = AppNotification.GetNotification();
                   if (notification != null && notification.ID > 0)
                   {          
                       //Gets all devices to send the above notification to         
                       List<IosDevice> devices = IosDevice.GetDevices(!isProduction);
                       if (devices != null && devices.Count > 0)
                       {
                           PushBroker push = new PushBroker();//a single instance per app
                           //Wire up the events for all the services that the broker registers
                           push.OnNotificationSent += NotificationSent;
                           push.OnChannelException += ChannelException;
                           push.OnServiceException += ServiceException;
                           push.OnNotificationFailed += NotificationFailed;
                           push.OnDeviceSubscriptionExpired += DeviceSubscriptionExpired;
                           push.OnChannelCreated += ChannelCreated;
                           push.OnChannelDestroyed += ChannelDestroyed;
                           //make sure your certifcates path are all good  
                           string apnsCertFile = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "../../Certificate/Certificates_Apple_Push_Production.p12");
                           if (!isProduction)
                               apnsCertFile = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "../../Certificate/Certificates_Apple_Push_Development.p12");
                           var appleCert = File.ReadAllBytes(apnsCertFile);
                           push.RegisterAppleService(new ApplePushChannelSettings(isProduction, appleCert, "135TrID35")); //Extension method
   
                           foreach (IosDevice device in devices)
                           {
                               //if it is required to send additional information as well as the alert message, uncomment objects[] and WithCustomItem 
                               //object[] obj = { "North", "5" };
   
                               push.QueueNotification(new AppleNotification()
                              .ForDeviceToken(device.DeviceToken)
                              .WithAlert(DateTime.Now.ToString())//(notification.AlertMessage)
                                   //.WithCustomItem("Link", obj)
                              .WithBadge(device.BadgeCount + 1)
                              .WithSound(notification.SoundFile));//sound.caf
                           }
                           push.StopAllServices(waitForQueuesToFinish: true);
                       }
                   }
                   Console.WriteLine("Queue Finished, press return to exit...");
                   Console.ReadLine();
               }
               catch (Exception ex)
               {
                   Console.WriteLine(ex.Message);
                   Console.ReadLine();
               }
           }
   
           static void NotificationSent(object sender, INotification notification)
           {
               Console.WriteLine("Sent: " + sender + " -> " + notification);
           }
   
           static void NotificationFailed(object sender, INotification notification, Exception notificationFailureException)
           {
               Console.WriteLine("Failure: " + sender + " -> " + notificationFailureException.Message + " -> " + notification);
           }
   
           static void ChannelException(object sender, IPushChannel channel, Exception exception)
           {
               Console.WriteLine("Channel Exception: " + sender + " -> " + exception);
           }
   
           static void ServiceException(object sender, Exception exception)
           {
               Console.WriteLine("Service Exception: " + sender + " -> " + exception);
           }
   
           static void DeviceSubscriptionExpired(object sender, string expiredDeviceSubscriptionId, DateTime timestamp, INotification notification)
           {
               Console.WriteLine("Device Subscription Expired: " + sender + " -> " + expiredDeviceSubscriptionId);
           }
   
           static void ChannelDestroyed(object sender)
           {
               Console.WriteLine("Channel Destroyed for: " + sender);
           }
   
           static void ChannelCreated(object sender, IPushChannel pushChannel)
           {
               Console.WriteLine("Channel Created for: " + sender);
           }  
       }
   
   }