Determining how many messages are on the Azure Service Bus Queue
var nsmgr = Microsoft.ServiceBus.NamespaceManager.CreateFromConnectionString(connectionString);long count = nsmgr.GetQueue(queueName).MessageCount;
It is called MessagesCountDetails.ActiveMessageCount.It returns the number of the Active Messages in the Queue. You probably have some Dead letter messages:
var msg = Microsoft.ServiceBus.NamespaceManager.CreateFromConnectionString(Settings.Default.ConnectionString);numofmessages.Text = msg.GetQueue(QueueName).MessageCountDetails.ActiveMessageCount.ToString();
have you looked at the Queue Description API? There's a property called MessageCount.
Here's the .NET SDK reference documentation page as well.