what if I have two iPhones and I use both but I want to add both phones on my device list. How does that work ?

If they are logged in to the same Apple ID they will appear. If they are logged in to different Apple IDs for privacy reasons you can’t view them, unless they are both part of a family sharing group.

Did you mean add both iPhones on a single Apple ID's device list? If YES then you can use the same Apple ID for both devices and both of your iPhones will then show up under the same Apple ID.

Axel F.