There is no API to increase the icon number. You need to save all the state on the server, since only you have the API to send the full number for display using the application icon. There is simply no other way. You already store the token of the user's device, so you can probably easily add the badgeCount attribute to any system used to save.
You will receive a notification if the user opens the application using push notifications using application:didReceiveRemoteNotification: in the application delegate. You can do whatever you want to inform the server about the current state of affairs. You can also do this in application:didBecomeActive: etc., if the user opens the application manually, and not through a notification.