Two-factor with only one Apple device...?
How does two-factor authentication work if I only have one Apple device (i.e. an iPhone) and no secondary device to help authenticate??
iPhone 11 Pro, iOS 14
The two-factors are your password and your Apple device.You do not need multiple Apple devices. But you should set up multiple trusted phone numbers in case of a problem. The phone numbers can be landlines, Android phones, friends' or relatives' phones. See information in Two-factor authentication for Apple ID - Apple Support