When do they stop the iPod Touch 7 updates?

Usually they offer security updates for a while - how long that is depends on what Apple wants to do. (If I were doing it I’d look and see how many of the devices are still active and then when the active devices reach a certain benchmark then stop, but I have no idea at all what Apple’s criteria is.)

I actually assumed it’d get 16 but apparently not. So is what it is.

The iPod touch was discontinued, because other companies can copy it. Apple does not want to be competitive with it, so they discontinued it. Let me know if there are any more questions.

We never know. iOS 12 just got another security update. You may get some more.

Lots of Apple devices, including some older iPhones, use (and cannot go past) iOS 15. We can only assume that Apple will continue to offer security fixes for these devices for some time.