static async Task TestPublishing()
{
    const int maxNumQueues = 20;

    AmazonSQSConfig sqsConfig = new AmazonSQSConfig
    {
        ServiceURL = "http://localhost:4566"
    };
    AmazonSimpleNotificationServiceConfig snsConfig = new AmazonSimpleNotificationServiceConfig
    {
        ServiceURL = "http://localhost:4566"
    };

    using (AmazonSQSClient SQS = new AmazonSQSClient("ignore", "ignore", sqsConfig))
    using (AmazonSimpleNotificationServiceClient SNS = new AmazonSimpleNotificationServiceClient("ignore", "ignore", snsConfig))
    {
        // Create a topic.
        CreateTopicResponse createTopicResp = await SNS.CreateTopicAsync("testTopic");

        for (int numQueues = 0; numQueues <= maxNumQueues; numQueues++)
        {
            // Create and subscribe queues to the topic.
            List<string> queues = new List<string>();
            List<string> arns = new List<string>();
            for (int i = 0; i < numQueues; i++)
            {
                string queueName = string.Format("testQueue_{0}", Guid.NewGuid().ToString());
                CreateQueueResponse createQueueResp = await SQS.CreateQueueAsync(queueName);
                queues.Add(createQueueResp.QueueUrl);

                // It looks with LocalStack special permissions are necessary.
                if (SQS.Config.RegionEndpoint == null)
                {
                     await SQS.AddPermissionAsync(createQueueResp.QueueUrl, "", new List<string> { "*" }, new List<string> { "SendMessage" });

                     int retryCount = 0;
                     while (true)
                     {
                        try
                        {
                            string subscriptionArn = await SNS.SubscribeQueueAsync(createTopicResp.TopicArn, SQS, createQueueResp.QueueUrl);
                            arns.Add(subscriptionArn);
                            break;
                        }
                        catch (Exception ex) when (ex is ThirdParty.Json.LitJson.JsonException)
                        {
                            if (retryCount++ == 10)
                                throw;

                            await Task.Delay(1000);
                        }
                     }
                }
                else
                {
                    string subscriptionArn = await SNS.SubscribeQueueAsync(createTopicResp.TopicArn, SQS, createQueueResp.QueueUrl);
                    arns.Add(subscriptionArn);
                }
            }

            // Publish a message.
            string msg = new string('x', 100);
            DateTime tm0 = DateTime.Now;
            PublishResponse pubResp = await SNS.PublishAsync(createTopicResp.TopicArn, msg);
            DateTime tm1 = DateTime.Now;
            Console.WriteLine("Num. queues = {0,2}  Publishing time = {1:F0} ms.", numQueues, (tm1 - tm0).TotalMilliseconds);

            // Cleanup subscriptions and queues.
            foreach (string arn in arns)
            {
                await SNS.UnsubscribeAsync(arn);
            }
            foreach (string url in queues)
            {
                await SQS.DeleteQueueAsync(url);
            }
        }
    }
}