Questions about Push Notifications


I would like you to help me to understand the following:
1)Is any limit to the number of devices IDs I can past in the query builder with "include" parameter?

I'm pasting them in the following format ["deviceID_1" , "deviceID_2", ... "deviceID_n"]

2) I need to register devices along with their user IDs, (eventually I want to send based on user ID, but I will map it to the device ID). My understanding, I cannot rely on the default device registration in this case (where the device id is inserted in to devices collection automatically upon launching the App). Am I correct?
If so, then I will need to register the device using REST service like in this tutorial ( My question is, assuming the user deleted the App, how I'm going to delete the device ID record from my custom table?
I know it can be done via unregistering the device. But how to trigger that? How to know that the user is deleting the App?

Many thanks for your help
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