How many devices do I have?
How many devices do I have on this line?
iPhone 7, iOS 15
Question marked as
Which line? How many Apple IDs do you have?
You can only know how many devices are logged in using an Apple ID.
Settings > [Your Name] > Scroll down [The lists of devices using your Apple ID are listed here]
At any given time you can log in with only one Apple ID on a device.