Google Analytics, not that hard.
You sign up for free, download their files, put them in your project, and each step you want to track, you can send to GA with some really easy code.
[GATracker trackPage😡"iphone/page1"]; something like that, i don't have any projects open, but it's to give an example. On each VC you want to trigger these tracks, you can do that, you can check how many active users etc.
Also, you can trigger things like you want in the appdelegate for example, when you start tracking, is to send the info to GA, not sure if this is build in, or you need to use some DeviceHW identifier component to check & send it to GA yourself.