Windows Phone 7 push notifications limited to 15 third-party apps, Microsoft clarifies0
However even applications that are not pinned to a live tile and use push notifications will also be counted. Basically, it all comes down to counting open channels that use the network connection. Now, that seems a bit more reasonable and brings the total of apps you can have to over 20, but it still remains a limitation.
Microsoft's Jaime Rodriguez argues that developers should hardcode the option to choose whether or not to use push notifications, so that users don't have to end up uninstalling apps to free up a network channel, but can rather just turn the notification switch on or off. Reasonable as it is, it sounds a bit like passing the buck.
Why was the limit set? The Microsoft team looked at the common user and came up with this number as a safe assumption. Were they right? We would vote against any limitations, but at the same time it is hard to disagree that the number of third-party apps used would hardly surpass 15.
Finally, Rodriguez claims that a better user experience is in the core of this limit. Good battery life, quality of service and bandwidth could have been the most valid reasons. Hopefully, with all the buzz surrounding this cap on apps, measures will be taken by Microsoft and newer versions will leave even less space for complaints.
source: Jaime Rodriguez blog