WNS or MPNS for Push notifications?
The 500 push notifications per day per subscription in MPNS only apply is you don't authenticate your service calls to MPNS. If you authenticate, there won't be any limits.
Such a high frequency doen't seem an appropriate use for push notifications for mobile devices, but I'm not sure about the alternatives.
WNS is the push notifications service for Windows 8 store applications.MPNS is the push notification service for Windows Phone devices.The service you should use depends on which OS you are targeting.
With the release of Windows Phone 8.1, WNS is now available for Windows Phone, too. Since convergence is the goal, prefer the new WNS API. The MPN API is still available for backwards compatibility though. The MPN layer in WP8.1 is actually a shim built on WNS. For more details, watch the Build 2014 session "Notification Platform Development on Windows".
You should use WNS, since Windows Phone 8.1 is using also this method for Universal Apps.WP 8.1 Silverlight Apps could use both.